I no longer have a Royale and haven’t seen a .25 Wildcat, but based upon other rifles, in my view, and if the regulator settings are indeed identical (there is often a small margin of error), the results tend to indicate a different air pressure patterns resulting from different balance of factors around size of transfer ports/dwell times etc; the principal contributing factor in this case likely being differing volumes in the outlet valve chamber. A slightly longer air wave is used more effectively with a heavier pellet, the lighter pellet being out of the barrel sooner and therefore unable to take advantage of the extra air.
I would be more interested/concerned about this issue from the perspective of air efficiency figures, to both improve such but also reduce muzzle noise and theoretically/potentially increase accuracy.