Right now, a title can only belong to 1 category. But many books (like romantasyor historical thrillers) straddle multiple genres. When a book is tied to only 1 category, it's not showing up on searches for the other relevant genres. For example, when you search best-selling romance books on Pango, Fourth Wing and ACOTAR don't show up, even though they are relevant to some romance readers.
Barnes and Noble, StoryGraph, and Goodreads all allow titles to be associated with multiple genres. See examples for Fourth Wing attached.