Why are branches in the Merged tab?

[Deleted user]

I'm asking this question because it doesn't behave the way I'd expect.

I expect all branches within the Merged tab to behind the develop (master) branch, as per the tooltip description when you hover over the Merged tab.

I have an example where a branch is showing that it is behind the master branch, but when I request a PR from the feature branch in question to the develop branch, there are commits.

For me this is extremely worrying, as the assumption that all branches in the Merged tab can be deleted could cause work to be lost.