It sometimes works the other way too. On a number of occasions I've had WT+ tickets cost less than WT at the time of booking (usually close to departure), due to there only being the highest fare class left available in WT, but lots of availability in WT+. I don't know if the BA revenue management algorithms ever spot these oddities and rejig accordingly?

I've seen it even more often on MH (which presumably has less sophisticated algorithms?), and recently had the choice of an F ticket for 1/3 of the price of a J ticket. No brainier!