Module release date and commit date?

Question

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?