My understanding is that the remote HEAD (the HEAD symbolic-ref in my Bitbucket repository) points to the head of the default branch. As such, HEAD in my Bitbucket repository resolves to the same commit as master (i.e. origin/HEAD and origin/master are the same for me).
Lets say I've created a branch "develop" and I want that branch to be the default branch. I select that as my default branch in the repository.
I would expect that the HEAD symbolic reference in my Bitbucket repository to be changed to develop and as such I should see origin/HEAD be the same as origin/develop. Instead, the reference does not change - it is still referencing master.
See also the following. Apparently changing the default branch in github manages the HEAD as I've described.