This is an interesting question, as it concerns the little-known matter of independent candidates in what were effectively one-party states. The short answer is that the "independents" are not only not independent in the most obvious sense (in that they are still effectively controlled by the regime's ruling Workers' Party of Korea), but also in that they are not without affiliations - it's just that they are not affiliated with a recognized political party and are instead nominated by one of the so-called mass-organizations.

The longer answer is that practically every communist state had ostensibly independent candidates taking part in various elections and securing seats, and North Korea is not fundamentally different from either the pre-perestroika Soviet Union, and the People's Republic of China to this day, in that it technically allows for candidates with no recognized party affiliation to take part in parliamentary elections, but effectively it amounts to no functional difference (from ruling party candidates) as I will try to explain.

Before I can begin answering the question in full, I need to outline the basics of North Korea's electoral system, such as they are on paper. North Korea's Parliament is unicameral, and is known as the Supreme People's Assembly (SPA), currently comprising 687 MPs elected for 5 year terms. All candidates are expected to be nominated by a party or a recognized organization, though on paper anyone could nominate a candidate. Nominations are then examined by the United Reunification Front (the Democratic Front for the Reunification of Korea to be exact, more on that later) and then by the Central Electoral Committee, which then allocates candidates to seats. The candidates in each assigned seat are thus unopposed as only one candidate is approved for any given seat (and in most cases only one is nominated), and are then considered by the electorate through a secret ballot UNLESS one chooses to vote against the candidate in which case the voter is required to self-identify in the allotted space on their ballot (which, predictably, nobody does).

This effectively offers a partial answer to the question - as every candidate must be pre-approved by the governing coalition (the Democratic Front for the Reunification of Korea), and the coalition in question encompasses every single recognized political party and mass organizations of all kinds (e.g. non-party bodies such as youth organizations, professional leagues, etc), AND the coalition is also dominated by the ruling Workers' Party of Korea (WKP), there are no truly independent candidates. This is not entirely dissimilar to the system that was in place in the Soviet Union and is in place in the People's Republic of China, among others. In the Soviet Union, for example, all of the candidates were fielded as part of a single "bloc of communists and independents", again, unopposed. In all of those systems there was, in effect, a ceiling to how far one could progress professionally, academically, and politically, without being a member of either the CPSU/VLKSM in the Soviet Union, CPC in China, or WKP in North Korea. The difference is that the pressure to become a ruling party member is recognized to be much more explicit in North Korea than in either of the other two examples. It is understood that in the late 80s anywhere between 12% to 15% of the entire North Korean population were members of the ruling party (in comparison to only around 4% and 7% in China and the USSR in the same period), matched only by the GDR and Romania.

Going off of that, it is not entirely unusual for "independent" candidates to take part in North Korean parliamentary elections - 1998 was certainly not unique in this respect and neither was it the "most successful" year for "independents." Depending on how one considers the specificities of the North Korean party system the current Supreme People's Assembly includes either 2, 3, or 8 independent MPs.

Here we come to the major issue at play. As you probably noticed, I didn't directly address the question of who those ostensibly independent MPs are (and why is there no definitive consensus regarding the number of those MPs at the moment). The answer mostly concerns the distinction between recognized political parties and mass organizations that I referred to earlier. It would be almost impossible to acquire actual in-depth information regarding individual MPs asides from major WKP functionaries (even precise election results are hard to come by), but in effect they all belong to either one of the three recognized parties (WKP, along with the Korean Social Democratic Party, and the Party of Young Friends of Cheondogyo aka Chondoist Chongu Party, who are both effectively controlled by the WKP), or one of the recognized mass-organizations.

Candidates nominated by those organizations are considered to be independent as they are not members of any recognized party, and would show up as de-facto independent candidates in the official statistics. For example the official communique regarding the first session of the 14th SPA notes that the elected MPs include "officials of party and power organs, administrative and economic bodies, working people's organizations, and those of various fields of science, education, public health, sports, literature and art, press and media, and also officials of the General Association of Korean Residents in Japan and organizations under it". Here one can note the discrepancy between the official North Korean records and foreign analysis.

In most cases this concerns the last body, the General Association of Korean Residents in Japan or Chongryon/총련 (朝鮮総連, or Chosen Soren in Japanese), which is a DPRK-affiliated organization representing ethnic Koreans or long-term (North) Korean residents in Japan. (There is also the Korean Residents Union in Japan/재일본대한민국민단, or Mindan/민단, which is larger and has ties to South Korea, that comprises most of the Korean residents in Japan without Japanese citizenship.) Senior officials from that organization routinely get nominated and subsequently elected to the SPA, but as they are not nominated by a party they would be officially considered to be an independent MP. By most accounts the majority of "independent" candidates nominated in North Korean elections over the last several decades were nominees of Chongryon, which is why in some Western records they can be wrongly considered party-nominated, though de-facto they are independent. There are at least 5 Korean-Japanese MPs in the current SPA, most (if not all) of whom are likely to have been nominated by Chongryon. I don't have access to the 1998 results to clarify this, but it is safe to assume that most of the 10 "independent" MPs elected that year were also Chongryon officials. The remaining "independents" can theoretically be members of various non-party organizations that are also part of the Reunification Front (such as the Kimilsungist-Kimjongilist Youth League, Socialist Women's Union of Korea, or the North Korean Red Cross Society, which is in effect a state body) but it is less likely as membership of those organizations largely overlaps with those of the 1+2 recognized parties. Another possibility is that an "independent" is nominated by a recognized religious organization, though those are similarly rare.