Sure. You have one "ground" for the resistor, and one for InAmp. Putting a voltmeter
across the resistor assumes ground is the bottom of the resistor, but the InAmp has
a different ground. Also, where does the input bias current go? Picture an NPN diff pair.
If you connect a battery between the bases, where does the base current go?
You have to have one ground only in your circuit and everything has to be referred to it.
Harry