I had just interpreted the rules incorrectly, thinking that B fares were ineligible for the 500-mile minimum. From delta.com:
* Discounted fares: 500 miles minimum or actual miles flown, whichever is greater, or
* Y, B, or M class of service: 150% of mileage
Because of the bolded "or" (emphasis not mine), it sure looks to me like YBM fares give you 150% of actual flown miles, without the 500-mile minimum. But that is not actually the case, as you said, and a dummy booking I just tried confirms.