I think this is because you're buying up to a B fare, which is what gives you the mileage bonus. The only exception would then be for a Plat, since they only have to buy up to an M fare, which (while less expensive) wouldn't give the mileage bonus. Therefore, if you're Plat and you really need the extra miles (and the fare difference between M and B isn't that great), you might want to go for B. I haven't tried this myself, so happy to be corrected. Since as a Plat you don't have any charges to do this via an agent, you may as well manage this with a live person to ensure you're getting what you want.