Mostly because they keep releasing tons and tons of opening books, and giving them all the same name would become a bit confusing.
I don't think there's much of a series idea behind the "Play the" books. The Starting Out books are supposed to be an introduction to an opening. Dangerous Weapons doesn't treat the opening as a whole, but has chapters on various new and interesting variations within it, so that if you already play that opening with either side you can probably pick a few of them to add to what you already play. "Move by Move" is supposed to feature some kind of question/answer system that tries to emulate what you would get from a session with a trainer.
What you should select rather depends on your level, what you already know, and what you want to play. Which series the book is in matters less, some of them are much better than others in the same series.
Regarding opening study, which books to select from EVERYMAN CHESS?
play the series, or starting out series, or dangerous weapons or move by move series.
Whats the difference among them? Are they for players with different ratings? Why different series regarding same opening from same publication?