When we do reviews we sometimes ask the author of a Diff to change his commit message to better explain what he was doing. When the author change the commit message the Diff title and the summary is not changed. The root problem is that if we forget to change them through the web UI, the revision will be landed with wrong commit message.
I hope I explained well the root problem, if not just let me know and will add details.
You can use --verbatim to cause this update.
When the local commit message differs from the web version, we can't always guess which one is right in the general case. We treat the web one as authoritative by default, since we know that it is expected to get edits (e.g., via Herald and via reviewers changing reviewer status), often has multiple editors, and is what most people are looking at.
I'm going to merge this into T2386, which discusses adding a warning about this to arc diff ("It looks like you updated the commit message summary or title, did you mean to use --verbatim?").