Tar produces different files each time

From the looks of things you’re probably being bitten by gzip timestamps; to avoid those, run

GZIP=-n tar -zcvf ...

Note that to get fully reproducible tarballs, you should also impose the sort order used by tar:

GZIP=-n tar --sort=name -zcvf ...

If your version of tar doesn’t support --sort, use this instead:

find source -print0 | LC_ALL=C sort -z | GZIP=-n tar --no-recursion --null -T - -zcvf ...