The ammeter is most likely connected thru a "shunted" circuit that does not carry the full load of the alternator. The ammeter is just reading charge and discharge not the full amperage of the alternator output. The shunted circuit is a parallel circuit to the actual charging circuit, it will tell you whether it is charging or discharging without having the full current passing through it. The fuses will protect the gauge and your vehicle in case the main charging circuit gets disconnected and all the current then is ran through your gauge.

Tom
1965 Chevy C10, 230 straight six, 2WD, 4-speed manual