I've reloaded Woodleigh softs and solids in 470, 500/416, 450 3.25" and 450-400 3" and there is ALWAYS a difference in the loads for the softs and solids. Woodies have a thick steel jacket covered by a copper jacket...but it's thin and the steel comes into play into the rifling. On the 470, to get the same velocity, it was as much as 2.5 grains....but generally 2 grains. The 450-400 is about 1.5 grains.
So not unusual at all to observe what you are seeing.

Gary