GoForSmoke:
I believe you. The appropriate analog pins let you measure electrical potential between them.That's not the same as feeding power to one analog pin without connecting ground between the board and the power source.
I find your use of language a bit confusing. Your second paragraph conjures up one-hand clapping. But I think you just mean the more usual mode in which a single analogue pin is used to measure a voltage by reference to Arduino GND.
...R