Module release date and commit date?
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.