I have seen that before too. But yes until you've allocated it, BA knows you are in 1A by status and as you say to transfer this as "I want this seat" then you could move and move back, but I don't think it makes much difference, you'll still get that seat. I don't always check shorthaul seating when I've got a lot on, and normally get 1A without checking MMB beforehand let alone moving around.