If you have any GP that's 05+, the charging system uses a current sensor to monitor battery discharge and changes the voltage accordingly. It's normal to see high 12's or low 13's when there isn't much load on the charging system. I have a HO alternator and I thought the voltage regulator was going out on mine until I figured this out.

If you're interested, I can post up GM's explanation of the charging system when I get home.