As mentioned, the slower burning powder is producing equivocal velocity but less pressure, even though it takes more of it in grains. Sometimes the slower burn rate will produce even higher velocities with the same or even less pressure.
This is very similar to a 30.06 using 4350 v. Reloder 22 for a 180 bullet. You can just about overfill the .06 case with Re22 and compress the load, and still have unremarkable pressures. Yet the 4350 may not even come close to the neck with a max charge and have noticeable pressure. Velocities can be very similar or noticeably different.