When the other tip already contains your tip in its history, a merge just moves your note; no commit is made unless you ask for one.

It explains why some merges leave no merge commit and what "non-fast-forward" means later.

Sometimes there is nothing to combine. Your branch has no new commits. The other one is simply ahead.

The other tip already has your tip in its history.

So Git just moves your note forward to that tip. No new commit. That is a fast forward.

Nothing was merged, because there was nothing on your side to merge.

You can ask Git for a merge commit anyway. Some teams do, to keep a record that the branch existed.

Check yourself. A merge produced no new commit. What was true of the two branches?