Only for the 6.x branch , the zip to download is totally different from the repository version:
http://drupal.org/project/google_plusone

Just now 6.x-1.0-rc6 and dev versions are the same, but not in the zip to downloads. If you download the rc6 version contains an old version (maybe a old beta? of a week ago?). Just look at the difference on the size of the zip between rc6 and dev.

I don't know since when it's happening.