It seems that the Distro search in Tasks actually searches by the OSMajor, and always has as far as I can tell. Perhaps we should either change the code to search for Distro, or change the label to specify OSMajor. I vote for the label change as that's a direct correlation to the actual search, whereas searching by Distro is not.
On Gerrit: http://gerrit.beaker-project.org/2705
This change is included in the Beaker 0.15.3 maintenance release: http://beaker-project.org/docs/whats-new/release-0.15.html#beaker-0-15-3