> Perhaps they refer to .zip instead of .tar.gz which as mentioned is > a less stable format due to the inclusion of the timezone.
Nope. I myself also faced tarballs checksum difference (even between few calls). GH support answered me (in TL;DR version) "that's because we've upgraded git on *some* of our nodes" (means, some other using older git), and "we've never guaranteed same checksums".
