Can't buy Payday 2?

Hmm, well I guess that makes sense then. But yeah, still a bit confusing.

I'm not sure why they decided to go that route (it is a tad confusing to people, I'm sure).

It's presumably for the same reason Super Street Fighter lists as Ultra Street Fighter- if they don't share the same base game entry on Steam, Steam apparently doesn't allow multiplayer connections [through Steamworks]; likewise, the base game apparently has to be given a singular name [as the different versions are in fact the same game entry, but with different associated DLCs]. Ergo, the most recent labeling of the game is shared across all DLC-pack variations of the base game.

In short, the publisher's three options were to rerelease the game so as to not confuse earlier purchasers, but then lose out on multiplayer cross-compatibility, or to have older versions of the game pick up the title of the new version, or to have the new version retain the title of the base game. The last one does make more inherent sense, but it wouldn't really avoid confusions much better than the current approach; after all, the real issue is how Steamworks (and an apparent lack of being able to separately label individual subs) limits publishers in being able to offer clearer labeling.