I would have to ask what multi meter are you using?
do you have another car battery to try that out with?
what was your initial tool that made you see it was over charging?

while I have never seen this before, and I don't see how it is possible, I won't rule out a bad battery. but typically when drawing voltage to run stuff in the car, that voltage would slowly drop not spike. lol.
So I would start with a suspect meter until proven good on another battery/car.