About ideal floated measurement
Clearly if ZH=ZL and ZHG=ZLG then the measurement will be immune to the common mode source.

In practice however, the construction of a typical instrument that has not been deliberately constructed to meet the above conditions will involve a nearly direction connection from the internal circuit ground reference to the external chassis or ground connection. This is the context of stating that typically ZHG>>ZLG since ZLG is nearly zero while ZHG is only due to weak parasitic coupling.

Edit: In the case where there has been a moderate attempt to isolate the internal ground from the external ground of the instrument, the simple fact that the internal ground structure is typically much larger than the positive side of the instrument, will introduce substantially higher parasitic coupling of the internal ground to the external ground creating an imbalance between ZHG and ZLG.

This is regularly demonstrated by freshman engineering students in the lab when they work on their first high voltage circuit, hoping to make a differential measurement with their scope only to garner a healthy shock or to let out the magic smoke. If the lab lights go out, it makes a fine teachable moment if the TA keeps his/her composure...