Voltage difference on DSO-150 'scope' and Fluke 87 multimeter
The issue was a faulty voltage supply to the scope. It was running off of 5V while it is specified to run off of 9V. I discovered this by checking step 4 of the assembly manual for the scope.
After adjusting my power supply to 9V, the base measurement still was off (but now by around +2V). Using the VPos Adjustment (top right, last page of the manual) fixed this and now my measurements make sense again.