I have a branch and I merged it into the master branch (via Pull Request).Then I un-ticked the box to not delete the branch. Now when I look at the branches this particular branch shows as 1 commit behind the master branch.
I don't understand this, how can it be a commit behind when the commit was what was merged from my branch into master? Is there a way to fix this so that it knows it's the same commit?
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.
First, from your description, it is clear that you are using a graphical engine for git. That can sometimes present issues.
Second, this message means that you have not pushed your commit and someone has made an additional commit in your current branch (and pushed their changes) after you last pulled and comitted.
To resolve the issue:
