This is a great question, and the answer has to do more with courtesy to the living than accuracy in terms. Several weeks ago, I wrote the following (amended here slightly):

The term "mythology" is problematic because it is used in radically different ways by different people. I taught "Mythology and Folklore" at the university level for over the span of four decades, and I found it useful to use this term exclusively for the recorded narratives of ancient people, narratives that were based on contemporaneous oral traditions.

According to this approach, "mythologies" are the written expression of oral narratives of the folklore of ancient people. This literature is populated with a range of supernatural beings and heroes. It includes etiological legends - stories told to be believed that explained the origin of things. The stories also included historical legends - stories about historical times, describing cultural heroes who confronted a range of opponents, often including supernatural beings.

I do not apply the term "mythology" to modern expressions of religion or folklore because the word can take on a judgmental tone. I had Native American students in my classes who took offense when other students asked about "Indian mythologies." The problem here was that these students often had parents or grandparents who were full participants in pre-contact belief systems, and "Indian mythologies" meant to them, "stories about superstitious if not silly beliefs." Consider the difference between referring to the Christian narrative as "the story of the Resurrection" as opposed to the "myth of the Resurrection." The second alternative implies that the Resurrection may be a nice story, but it is not to be taken as truth.

This standard regarding the use of the term "mythology" is not universal and it is not without problems. By applying it to the belief systems and traditions of pre-conversion Scandinavia, we run the risk of stepping on the toes of Neo-Pagans - and that's not fair. By not applying the term "mythology" to other active religions with narratives grounded in the fantastic, we are implying something about those modern faiths that may not hold water when viewed objectively. Hence your valid question, which is best answered with the desire not to be cruel or to start an argument with living believers.

Over the years, I put together an Introduction to Folklore for my students, borrowing heavily from my mentor's similar work - Sven S. Liljeblad (1899-2000); for more on him, see my article Nazis, Trolls and the Grateful Dead. The following from the Introduction deals with various terms used in folklore, and addresses how folklorists tend to lean into the term "mythology" - although there is no consistency even among folklorists!:

European folklorists, following the lead of the folk themselves, have long recognized two forms of oral tradition, Sagen and Märchen, legends and folktales. While there are many other forms of oral tradition, legends and folktales stand in opposition to one another, yet share a great deal. In reality, lines can blur.

Legends – or Sagen as the profession often prefers – are generally short, single-episodic stories told chiefly in the daytime. More importantly, the teller intended the listener to believe the story. Legends often have horrible ending to underscore the story’s important message. Many of them are, after all, meant to be instructive, to serve as warnings in some way. These types of stories are not necessarily long-lived. Their point is to reinforce and prove the legitimacy of a belief. Nonetheless, some legends take on a traditional character, can become multi-episodic, and migrate over considerable spans of time and space.

Folktales – or Märchen, again using the German, technical term – are longer stories with more than one episode. They are restricted, in theory at least, to evening presentation. A folktale is not to be believed, taking place in a fantastic setting. The European folktale also requires a happy ending, the cliché of “happily ever after.” Any given folktale can be told with considerable variation, but they are traditional in basic form, and folklorists have spent decades tracing the history and distribution of these stories.

Besides the legend and the folktale, there is also the folk ballad, a specialized form of oral tradition that, like the others, incorporated a wide range of beliefs. The ballad had roots in medieval Europe, combining narrative and song. The ballad usually focused on a single incident, and it almost always emphasizes action.

Something also needs to be said here about myth. People use this term awkwardly. In a European context, myths tend to be the artificial constructs of ancient and Classical-era priests or literate people who sought to weave folk traditions into a comprehensive whole. The exercise often had political purposes, designed to provide diverse people with a single set of beliefs and stories. By reconciling similar traditions, the shared culture of these groups could be seen as more important than the differences, justifying the central rule of the king and his priests. Myth is also a way of organizing and reconciling folk traditions, which by their nature can be contradictory and highly localized. Myth tends, however, to make gods of supernatural beings, giving those powerful entities a status – for modern readers – similar to the Judeo-Christian-Islamic God, even when this comparison is not justified. Of course, it is also important to point out that myths were stories that were told – and then written down – and they were different from religion itself. Many myths were simply the shared cultural inheritance of a group of people.

In general, the word myth is best set aside when discussing more recent folk traditions, recognizing its proper status as a literary genre. Nonetheless, ancient documents recording myths can assist in understanding the history of various stories and beliefs. The authors of these texts were, after all, the first folklorists, and they were the only ones coming close to practicing the craft at the time.

Some folklorists carelessly use the term myth to denote those legends that deal with a fantastic, remote time. This primal era saw the creation of many familiar things such as day and night, fire, animals, people, mountains, and all other aspects of the present world. Folklorists properly refer to these stories as etiological legends explaining the origin of things. Sometimes, however, people interchange etiological legends with the word myth. The problem with this is that “myth” can imply something that is inherently wrong, linked to “primitive” superstitious beliefs. When the term “myth” is used for the folklore of existing cultures or for the traditions that were viable only a generation or more ago, it can take on an insulting, derogatory tone. It is best to reserve the word “myth” for ancient and Classical-era texts.