Another Weird Voltage Thing. Maybe Headlight Problem

I recently put a digital voltage meter in my car so I could keep close tabs on it. I did this because I run a stereo with a lot of wattage. I'm running a 200 amp alternator and zero gauge wire with two optima red top batteries. After doing this I noticed something odd.

Now here's the weird part. My stereo can draw about 150 amps at full output. To test for any discharge at high output I revved the car up, cranked the stereo, and ran the A/C to see what would happen to the voltage. It stayed right on 14.4 even with all that draw. I was impressed that the alternator was keeping up no problem. Then I realized at night with both A/C and stereo off, when I turned the headlights on the voltage would drop to 13.7 and wouldn't go any higher I started messing with things and no matter how much crap I had running the voltage would always stay at 14.4 volts. Also if I had nothing running it would stay at 14.4 volts. Then if I turned the headlights on it would always drop to 13.7 volts. Also if I run everything and the headlights it still stays at the 13.7 volts. I wouldn't think our 55 watt bulbs draw anymore than 20 amps. That makes me think there has to be something funny going on with something in the headlight wiring. I'm hoping one of you guys has an idea as to what would cause this. I would appreciate any help.

I thought that might be the case. But if I have the headlights on and nothing else the voltage will be at 13.7 like I said, then when I turn the headlights off it jumps up to 14.4 with nothing on at all. It is really driving me nuts. You are right that the voltage should drop a bit when everything is fully charged. It will do this if I leave everything off for a while. Normally it drops a tenth of a volt at a time and bottoms out around 14.1 or 14.2 volts though. The drop with the headlights is instantaneous and much more of a drop. It's kind of driving me crazy.

Anybody else? I could really use some help. This voltage things is driving me crazy . I checked the grounds on the headlights and they look fine. I was also wondering if it could have something to do with the headlight switch. I'm open to any ideas.