You don't seem to understand airline revenue management and nesting booking classes. If a flight has 8 first class seats and none have been sold, then the inventory would look like this: F8 A6; essentially the airline's RM team is thinking that they can sell 2 seats at the full-fare F and hence why they are limiting the discounted A bucket to 6 seats; however, if they get lucky, they want the F bucket to have the ability to sell all 8 seats in such.

Thus, what the OP is saying is that with F6 on flights that don't appear to have any bookings, BA is capping the F cabin to 6 seats basically.