Sorry you weren't around when the re-naming process happened. BTS (ie: Beneath the Surface) is the new name for PCS2 (Project CS2) because, honestly, the title "Cave Story 2" Is so abhorrently overused, no subtitle can make it any better. (And also to follow in the naming steps of Jenn's Journey and Jenka's Nightmare)
Apparently not having "Story" at the end of your title makes it inherently better. (Discussed via skype)