Jeff: The bulb failure sensor measures an imbalance in current not voltage to any set of lamps. Your difference in voltage at the bulbs indicates that there is different current at each bulb. Voltage drop is normal and is dependent on current flowing which of course should be the same for each bulb. Therefore if one bulb draws more current than the other because it is older, different manufacture etc. that failure light will come on.
First, change both bulbs and install a matched set. That should solve the problem. If no then look for bad connections around the bulb with the higher voltage drop. Check supply and grounds.