I think the FF card is your answer. Has happened to me a few times, seems they first the FF cards going gold - silver - regular, if there`s no one with gold, then the silver gets checked then regular.
They seem to have some sort of check against your name at the gate for this, or they can run a query. One of the times I got upgraded at LHR, the Lady told me it was because of the color of my card.
Rgds