What's wrong with this 1996 Ford Ranger? Charging issue.?

My wife brought it home and said the battery light was on and it was making a strange noise. ( I replaced the battery and alternator 10 months ago. ) The alternator was buzzing, so I took it to the parts store. They said it was charging at 30 volts, and was defective. I installed a new alternator but the battery light is still on. I have 12.1 volts from the battery.

I drove it around for about 15 minutes with the lights on. I got it back and the battery was still at 12.1 volts, but the voltage gauge in the cab says low. Shouldn't it be reading closer to 13 volts?