The motor must be warmed up and idling in neutral or park and the car must be on level ground. Dexron/Mercon seems to have a high thermal expansion factor, too, so if the car has been driven hard in AC temperatures, the dipstick will read overfull even though it's OK.

If you checked it with the motor off and the car had been sitting for a while, the fluid SHOULD be way above the full mark, because that's not the way to check it properly, although I am in the habit of filing a little notch in the tranny dipstick of everyone of my cars to indicate what I have found to be the normal get-up-in-the-morning transmission level, just for a quick check in case there's any question about whether there's a reason to do a proper check before setting out on the road.