They are rpm dependent, but not *more* rpm dependent. The vac pad output simply turns too slow for the alternator to generate everything it's capable of. Look at the smaller, 8A B&C permanent magnet alternator. That alternator is actually a 20A alternator, when turned at its design rpm. The larger model is almost certainly a 60A model (might even be higher; I haven't looked closely at it). Both models are overdriven at least 2-1, often even ~3-1 to crank speed in their native environments. The low shaft rpm is the reason a 20A alternator only produces 8A at typical Lyc cruise rpm, mounted on the vac pad.
If the vac pad output is driven at 1-1.3, then anything near idle will have the alt turning <1500 rpm, and at cruise, ~3100 rpm. In its original design element, it would be turning >2000 rpm at design idle, and at normal engine operating rpm, it would be >5k rpm.
Not unlike a piston engine, alternator output is rpm dependent.