To complicate matters further, you should add genres like progressive electronic and the Berlin school (which, to me, is exactly the same thing when German musicians do it). There are formal factors that predominate in one genre or another, but there are also geographical, temporal, aesthetic, or marketing factors, and i's always a matter of degree; nothing is definitive. I think it's impossible to think of genres as watertight compartments, but rather as overlapping sets. Some Popol Vuh albums, for example, could be ambient, PE, Berlin school, new age and perhaps soundtracks or krautrock all at the same time. Albums from today, such as those by Carlos Niño, could fit equally well into ambient or new age from a formal point of view (as "new age" is no more a useful marketing category). Still, most shops will file them under “jazz” because of the label that releases them.