UserA has a repository (Repo1) they'd like to transfer to UserB. Prior to this point, UserB had explicit permission to this repository at http://bitbucket.org/usera/repo1/admin/access. When the repository transfer takes place from UserA -> UserB, the permission will remain. This effectively means that UserB is now both an owner, and has explicit permission to the repo. This also will cause UserB to have 1 less available user in their account.
The UI of the repo access page also won't show this, but the Manage Account screen for UserB will. https://bitbucket.org/account/user/userb/plans-and-billing/ would show that userb also has "Direct access" to repositories twice in that list.