When you pull specific branch, you (by design) pull branch's HEAD and all it's ancestors
Mergeset, which merge this head into default branch (if exist) is not
- ancestor of branch-head
- changeset in branch
<branchname>
and due to these two reasons can not be pulled.
It's not a problem, but correct and expected behavior.
If you asked "Why I didn't update to head", you have to re-read hg help update
If no changeset is specified, update to the tip of the current named branch
(i.e if your . was in default branch before, hg up will not change branch, you must hg up <branchname> for cross-branch updating)