Simply put Microsoft is the worst company at naming stuff. Even when they come up with a good name for something, they'll name 3 other totally different products the same thing to maximize confusion.
Series is a real weird word to use there. But it also doesn't help that the versions are extra complicated because with "PC-like compatibility" in everything after the Xbox One playing just about the entire same library you need a bit of a matrix to figure out which is best for you if you don't care about the "latest and greatest".
Weirdly it only has 8GB of internal storage...
Compare that to Playstation: 1, 2, 3, 4, 5.