Github Comparison View for 2 branches is incorrect?

Question

When I do a Github comparison view between master and another branch A, Github seems to be comparing the HEAD version of A with an old

Answer 1

Is there a way to diff the HEAD of master against the HEAD of Branch A in Github?

[EDIT: Nope, I was wrong. I managed had a URL which looked like it would be doing the right thing and the diff on the page looked right, but that was an accident. If you really want to know more, check the history of this answer.]