The problem there is that while the MCU itself on boards like the Mega and Due has a separate ground for the analog inputs, the boards (such as the Due and Mega) have one common ground plane for everything.
And if you get a decent current through any ground pin to the ground plane on the Due/Mega, it changes the readings.
On a Mega it's not so bad as your Max reading is 5V, whereas due to the lower 3.3V Max on the Due, the changes are more noticeable.
And if you get a decent current through any ground pin to the ground plane on the Due/Mega, it changes the readings.
On a Mega it's not so bad as your Max reading is 5V, whereas due to the lower 3.3V Max on the Due, the changes are more noticeable.