The most recently announced Kingdom Hearts release is Kingdom Hearts HD II.8 (er, 2.8, WHATEVER). That's kind of an odd title. But, according to Tetsuya Nomura, there is a reason for it. A confusing reason.
Speaking with Famitsu, the Kingdom Hearts designer explained that there are various meanings contained in the naming. "This Kingdom Hearts Dream Drop Distance compilation follows the previously HD work 2.5...and is worthy as taking its place as 2.6," said Nomura.
Continuing, he added that the thinking is that the absence of Sora in Kingdom Hearts χ and Kingdom Hearts Unchained χ as well as the χ series in Kingdom Hearts χ Back Cover is the number zero.
"Because Kingdom Hearts Birth By Sleep is 0.1, this is how the 0.2 came to be put in the latest Kingdom Hearts 0.2 Birth by Sleep," Nomura added.
"Thus," Nomura said, "the 2.8 number appears from 2.6 plus 0 plus 0.2."
Well, I'm glad this makes sense to someone! Especially to the person who's making these games. That's important, you know.