It really isn’t. The Dark Kingdom and Black Moon arc were produced as one 26 episode series. I guess it doesn’t really matter in the grand scheme but its having those two arcs grouped together and then having Infinity arc labelled season 3
Inconsistency thy name is Toei. This is yet another, albeit smaller, way in which they royally screwed up this adaptation.
The entire thing should have been called Sailor Moon Crystal with the name of each arc as a subtitle. The manga ended decades ago. The terms and names are all set in stone and familiar to fans to whom this adaptation is supposedly targeted towards. It's a no-brainer.
Instead of "Sailor Moon Crystal" and "Sailor Moon Crystal: Season III", it should have been named the following:
Sailor Moon Crystal: Dark Kingdom
Sailor Moon Crystal: Black Moon
Sailor Moon Crystal: Infinity
Sailor Moon Crystal: Dream
Sailor Moon Crystal: Stars
Simple. Clean. Consistent. Faithful. Appealing.