Unexpected/unexplained difference between git pull --rebase and git rebase

Hello,

I'm seeing unexpected behavior between "git pull --rebase" and "git rebase" commands, which are supposed to be (and always described as) synonymous:

git pull --rebase upstream our-branch-name

and

git fetch upstream git rebase upstream/our-branch-name

We have a situation where the upstream/our-branch-name was rebased, to incorporate changes from master. Somehow, the person who did the rebase discarded a merge commit:

634b622 Sue Merge pull request #254 from bob/B-07290 bc76e5b Bob [B-07290] Order Parts Ship To/Comments

became:

c1452be Sue [B-07290] Order Parts Ship To/Comments

A developer who had a local branch tried to rebase their work (a single commit on top of that feature branch).

At the moment, his now-out-of-date branch looks like this:

92b2194 Rick B-07241 634b622 Sue Merge pull request #254 from dboyle/B-07290 bc76e5b Bob [B-07290] Order Parts Ship To/Comments

I've done some debugging, and the above "git pull" command generates the following and sends it to eval():

git-rebase --onto c1452be62cf271a25d3d74cc63cd67eca51a127d 634b622870a1016e717067281c7739b1fe08e08d

This process works perfectly. The old commits are discarded and his branch now correctly reflects upstream/our-branch-name, with his single new commit at the top.

However, if he runs the "git rebase" command above, several of the commits that have changed hashes (they've also changed patch id slightly, because during the rebase someone fixed a merge conflict) are treated as new work, and git tries to re-apply them and we get tons of merge conflicts.

The git rebase command above is trying to rebase onto:

revisions = c1452be62cf271a25d3d74cc63cd67eca51a127d..92b2194e3adc29eb3fadd93ddded0ed34513d587

These two features should work the same, yet one is choosing a different commit hash than the other.

If this is not a bug, I can't find anyone who can explain what's happening. I'm using git 2.2.1 on mac, but other people on our team have a variety of older versions and we're all seeing the same result.

Thanks!