If the POU is designed to clear space in the lower cabin whilst giving BA some revenue then I imagine they offer it to a random selection of EC members, perhaps those with a card ending in an odd number or those who are born with the moon in aquarius. The fact that you were born on the wrong day meant you did not get the offer and the upgrade my ticket charge was the price differential between the 2 classes of ticket.
With the POU being described as a special offer, do they have to offer it to everyone? Wouldn't be special then would it?
I would like to point out that my background is in Sales & Engineering, I am therefore probably not best placed to comment on the precise ins and outs, just my 2p.... 