Actually, it's not true that the Finder version number will always be the same as the Mac OS X version number.
The Finder and Mac OS X version numbers aren't always the same.
But it IS true that for any given Mac OS X version, the version of the Finder will be a particular version number (but not necessarily the SAME number).
So when you upgrade to Mac OS X 10.7.3, the Finder was 10.7.3 because a Finder update was part of of the 10.7.3 update. And sometimes the Finder version get's ahead of the Mac OS X version.
But apparently there were no Finder updates in Mac OS X 10.7.4, so the Finder version remained at 10.7.3.