Small Arduino boards, like the Nano, have a simple USB backflow protection diode instead of a p-channel mosfet (Uno, Mega). That schottky diode causes the ~0.4volt drop of the USB supply, and a less 'solid' supply voltage with load changes (flashing LEDs, etc.).
If you use an Arduino as a voltmeter, it usually is better to use the internal 1.1volt Aref, and drop the 'voltage to measure' to <=1volt with a voltage divider.
Another way is to use the 3.3volt supply as reference, by linking the 3.3volt pin to the Aref pin.
MUST set Aref to EXTERNAL if you do so. Leaving it on default will damage the Aref circuit.
The 3.3volt supply of a Nano is not as good as on the bigger boards, because a Nano shares it with the USB<>Serial chip. The Uno/Mega has an independent voltage regulator.
Leo..