It sounds like you have an issue with a floating ground line. Check that there arn't any broken ground wires. Check that they all have a clean (paint and crap free) connection to the frame.
Disconnect the headlight to eliminate this as an issue. Take a measurement, then re connect it and do it again.
I know that when Ive had strange readings like this (in a part life as an elec. tech) it usually could be traced back to a faulty earth connection.