Seconding what Steve said, I think a cu. ft. measurement is considered "useable capacity". Reading between the lines a bit, useful capacity, even though the tub may be identical between various models, depends on what agitator is used. I believe that as efficiencies of agitators improves, useful capacity increases in the same space.
I believe this is why we saw measurements of 2.7 cu. ft. on older large capacity belt drive Whirlpools and Kenmores, and 2.98 with the same basket equipped with a DA. Fast forward to today, I similarly think that is why a DD with straight vane comes in at 3.0 cu ft., a basic Dual Action at 3.2 and the fancier DAAs at 3.5 - all with the same tub and basket.