Just because a fare is refundable doesn't mean that it is full-fare. Generally a B fare is a discounted refundable coach fare (not 100% sure about Delta's B fares). But since it is not "Y" is requires the discount fare upgrade mileage.

Kudos for AA to recognizing this confusion. They could B fares the same as Y for the purpose of upgrades (mileage amounts and complimentary upgrades for Executive Platinum top-tier elites on full-fares).