Remember that there are two measurements to electricity: Volts AND amps. The volts are read off the battery, essentially. Takes a while to drop off. BUT: the amps are the alt's output. OR: Volts are the potential required to make electricity flow in the wire, like pressure in a water system, but amps are the actual measurement of that flow, like gal's per minute in a water system. The battery stores trons until needed for starteing and such, and the potential across it stays fairly constant for a wide range of available amps contained within the battery. It's only once the supply of trons in the battery starts to run low that the voltage drops. SO: If the amps are low, the lights come on because the system is set up to check amps, not volts, but until the battery output voltage drops a bit, you won't really notice any change on the voltmeter. If You're running on the battery, tho, it won't be long till you see a drop. Try turning the lights on high beam, any other loads you can on the system, THEN check the volts. I bet it get's lower pretty quick.
Hope that helps a little
