If you are in the delta screen of the UI, it is possible to look at the existing changes. The info box states that I am on the default branch, even though I am on a separate named branch. Typing "hg branch" shows the correct named branch.
This can be very confusing, especially after cross-branch merges.
I believe that the info box in the delta screen should show the correct branch, as if you had typed "hg branch".