I've seen this in my 05 Grand Prix GXP when I had it. I discovered as well that the charging varies based on the demands.
An OBDII adapter and the 'Torque' app allows me to see the commanded voltage. Like stated earlier, dropping down does not mean something is wrong.
Also, not sure with the G8's but on the GP GXP there was a plug that could be unplugged on the alternator. With it unplugged, it would come up to the 14.7 voltage or so, and stay