If you upgrade TBB by overwriting an old one, it reads the old extensions.torbutton.updateNeeded
The check for a new Tor version opens a file on the file system for version information comparison.
There are scenarios where this file may be written to by an old version, or a exiting slowly version, as part of a TBB upgrade. (Or other scenarios where users do things totally unexpected and not recommended.)
Thus: when checking for newer versions, an old version is referenced, which will always claim to be out of date despite not being so.
If the version check including the version of the build of the checker/writer somehow this condition would be avoided. Note that keeping the version of the author/checker in the file itself could lead to race conditions requiring some thought, while a version in the file path would not conflict. (e.g. old and new write to version.txt compared to old write to git-abcdef12-version.txt and new write to git-f00ff00f-version.txt)