When you create a new branch, you choose what branch it should be cut from. It then redirects you to the branch page for that new branch (e.g. bitbucket.org/user/repo/branch/new-branch-name).
On that page, it will by default compare it to the default branch of the repo (usually
master). Instead, I think the branch creation page should populate the
dest query parameter to match the branch you chose on the previous page, so that it compares to the branch you'd expect it to.