Strange VOM Issue

So, I'm doing some wiring repairs on James' 63 Dart now. It had a pretty good meltdown behind the cluster. I have pretty much got it all repaired. Went to hook the battery up to test for power. My Fluke meter's battree is dead, so I broke out my Bosch VOM and started testing. I tested the main wire coming in and got 18.6V. So I immediately opened the hood on the Valiant and got 18.6V on her battery. Bullshit. So today I got a new 9V battery for the Fluke and I have 12.5V on Vixen and 12.4V on the 63 Dart. What can make the Bosch read like that? Is it possible to adjust it maybe?