[pekk]

I wouldn't read too much into changes of version numbers. vim was being updated continuously.

[garou]

The versions numbers talk about the changes happening and compatibility. There's just a few new features in the 7.3 [1]:

Persistent undo, More encryption, Conceal text, Lua interface, Python3 interface.

[1] http://vimdoc.sourceforge.net/htmldoc/version7.html