Releases should have ratings.
Changing the rating of one release changes the rating for all the rest of the releases in a release group, and for the release group itself. Try to rate two different releases in http://musicbrainz.org/release-group/a40cc0a8-2295-30bd-90a7-5a4ec0869235 (with different ratings, obviously enough) and then refresh the first one.
This might be by design. In that case, the design is terribly confusing and we should try to find a more clear one.