And, indeed, if you use BA Miles to upgrade to Club ... you find there are hardly ANY seats available on a 744 other than the ones they've allocated. The worst ones, of course.
Pay full price for Club, and all of a sudden there are seats everywhere on both decks. It's called
market forces, as F3G implied, and Gold Cards get priority over cheap-skate proles like me who are Blue Card, and who sometimes book PE and then upgrade with Miles.