This happened to me as well @Menno , I think it's a "feature" of GitHub and will likely come up again @Sebastiaan
Let me explain...
I forked when v10/contrib was the default branch.
After I did that, the v11/contrib branch was created and set as the default branch for the Umbraco repo (but not on my fork).
I later used the magic button in GitHub to "sync" my fork, this synced my fork's default branch with Umbraco's new default branch.
So my v10/contrib branch is actually tracking v11/contrib, and that v11/contrib branch doesn't exist in my fork.
This kind of makes sense because branch names don't really mean anything when you're working across repositories.