It's not true to say that BA give seats away by allocating them for redemptions.
Anyone with BA miles, even tesco whores have paid for their miles and BA is the major winner.

Fair enough,they get more for straight sales,but I should imagine most people these days in F are either on an offer of some kind,an MFU ,a full redemption or an op up on the day.
The latter explains a lot as F redemption availability may not necessarily depend on what is sold in the F cabin but what gets sold in all other cabins.

BA's revenue people who look after what seats are available for redemption chop and change all the time on availability from what I can see,sometimes feast sometimes famine.
They have their own methods.

Availability obviously tends to usually get better on flights that are not selling well within a month of travel,it seems.That appears to be the fallback they apply once they run out of fare paying possibilities.