First/oldest entry in reflog dropped

Can someone explain the behavior in the execution below?

# I expected this reflog...
$ git branch tmp
$ git reflog show refs/heads/tmp
b60a214 refs/heads/tmp@{0}: branch: Created from master

# ... and this one as well...
$ git update-ref refs/heads/tmp HEAD^
$ git reflog show refs/heads/tmp
7d1a0b8 refs/heads/tmp@{0}:
b60a214 refs/heads/tmp@{1}: branch: Created from master

# ... but why is the first entry (i.e. "branch: Created from master")
# dropped here?
$ git update-ref refs/heads/tmp HEAD
$ git reflog show refs/heads/tmp
b60a214 refs/heads/tmp@{0}:
7d1a0b8 refs/heads/tmp@{1}:

If the ref is updated once more (to e.g. HEAD^^) before being moved back
to HEAD, the first entry will be shown in the output.

If this is a bug, it seems to be in reflog, rather than in update-ref,
because the first entry does exist in .git/logs/refs/heads/tmp.

Thanks,
Martin