The kernel numbering system changed when Linus moved over to v3. Basically the switch from 2.4.x to 2.6.x was the last major change to the Linux kernel. Since then, all the 2.6.x releases have been pretty standard incremental updates. So a few months ago Linus decided it was time to switch to v3 - just to round the numbers off (it wasn't a big nor noteworthy release - it was just an excuse to round the numbering off). So now the the second digit is what used to be the 3rd digit in the 2.6.x releases.