Basically the first number didn't really have any meaning, so it was removed, so the major versions (which I believe are released once a year) are:

9.2 (released in 2012)

9.3

9.4

9.5

9.6

10

11

12

13 (about to be released this year?)

That's still a lot of versions in between, each of them as new functionality and requires converting the database to upgrade.