As far as I can tell the 5706 simply hasn't been pushed to the auto-update system (yet). Not sure how you could get your 5691 to detect 5706?
Edit: okay never mind, the update system actually advertises 5702 but the setup file is 5706, hahahaha '-_-
Also note that the /checkupdates command isn't exactly the same as a scheduled auto-update, Plus! 5 hasn't been published (at all) through auto-update yet. Just FYI