Module release date and commit date? I can't understand somethings about commits.

For example, for the Webform module, in its project page it is written that last version is "7.x-3.15 2011-Dec-08," but in the commits page, it is written that there is commits to 7.x-3.15 in 25 February 2012.

I also checked the releases page; one of them has been released on December 2011, and another has been released on February 2012.

Does it mean that these versions were commited to git, but not released?
So it was an unstable version?
Is this usage common for Drupal module releases?
It is common in pretty much every software project. It makes no sense to release a new version for every single change unless it is critical enough (e.g. security fix). So releases are e.g done on a regular schedule (Drupal core does one every month now).

It is common practice to list the issues that where implemented/fixed as part a release in the release notes.

There are usually multiple commits every day on Drupal core, imagine there would be a new version every time.

What's somewhat uncommon with Webform is that it they don't list their development snapshot releases on the project page. Those are automatically updated twice a day if there have been any commits. In case of Webform, that snapshot still exists, but, according to the description, should only be used for testing recently fixed issues: http://drupal.org/node/730862.

The quality of development snapshots can heavily vary between projects so it's generally not recommended to use them. One exception for example is if you encounter a bug in a module then it often makes sense to test the snapshot first if there is a newer version than the last release.