Because Koreans count their birthdays in two ways. In one way, they count it from their actual birth date. That's the Western way to count age. But the traditional way to count it is from Korean new year. Korean new year is in February. So for someone born on January 8, just a month later, they're going to be said in Korea to be "1 year old". And then on their next birth date, they'll still be counted as 1, until the next February, when they'll be counted as 2. Everybody in Korea ages up by one year on the same day in February.

So it's not uncommon for Koreans to use both, and you'll have to ask for clarification if you want to know the true date that they were born on.

Kim Jong Un's birthday is known through North Korean press reports, and so is his age. So we know he was born on January 8, but we don't have clarification of the year he was born because we don't know if North Korean news reports have been reporting his age the Western way, the Korean way, or a mix between the two, so he could have been born in one of three years.