Alternator amp rating.

Alternator only will supply what is demanded. It doesn't put out 75 or 100 or whatever unless there is equipment calling for it.
Alternators capacity to produce power goes up with rpm. The 'rating' of Alternators by the aftermarket is based on whatever, but if they are somewhat honest, its usually near maximum and based on 14 something Volts. Changing the voltage is one way to play games with the 'rating'.
The ammeter is a monitor of the battery current flow - NOT the alternator output. Having a high rated alternator will not increase the current the ammeter sees unless the battery has been really run down. If real lucky, the high rated alt might reduce the loads on the ammeter IF it makes more power at idle speeds. Some do, but others trade off low speed performance for better high speed output. When an alternator can't supply enough power at idle, then power is drawn from the battery.