From what you've described, it seems the "one commit" is a merge commit.
This is due to how git works: depending on the steps you performed to get your code merged, it will or will not create a merge commit in the target branch. Check your git history to confirm it.
If that is the case, you can safely ignore it. In a typical git workflow, your "feature branches" are short-lived, getting deleted when merged, so you won't even notice this difference.
If you don't want a merge commit, merging with fast-forward won't produce one, but you may need to rebase (or squash, depending on your preference) the source branch before the merge, which will change your branch history to allow a fast-forward.
Since you've tagged
Azure DevOps in your question, you can avoid the merge commit using the "Squash merge" or "Rebase and fast-forward" PR merge types.
P.S.: Also, if you are new to git, be extra careful when rewriting history. Things can get messy if you don't know what you are doing. I strongly recommend getting to know git better to get the most out of it.