If its an older 10SI alternator with the two wire plug in addition to the thick red wire,one of those wires on the plug "senses" the voltage and "tells" the regulator how many amps to deliver--if that wire gets corroded somewhere and it drops the sensed voltage,the alternator will put out more amps & volts because it is getting a signal the battery voltage is low..
I also dont rely on stock dash gauges,especially the volt meter--one on my truck sometimes wont go barely out of the red "low" side,especially after a cold start which takes a lot out of the batteries--but a check with a multimeter shows it is charging normally,and I can hear the alternator whining some,telling me its putting out a healthy charge..
--if I push on the dash or smack it,sometimes the volt gauge needle moves back where it belongs--poor connections at the printed circuit cluster or the gauge itself maybe--truck is nearly 35 years old,so some electrical gremlins can occour..
...I have replaced alternators for nothing because I believed the dash gauge on more than one GM vehicle I've owned..my '80 El-Camino's gauge said the alternator wasn't working so I put a known good one on it,and it was exactly the same...
I pushed in on the dash cluster,and the gauge went up 3 volts!..I jammed a penny between the clear plastic cover and the gauge and that fixed my low voltage readings..
