Just checked on this hot day and found some registers at 56F and others at 66F. There are 4 ducts, all the same size, coming from the plenum. All the registers on the 2 ducts closest to the coil are all at 56F. All the registers on the 2 ducts further from the coil are all at 66F. The ducts are only a few inches apart. The total distance from the coil to the registers doesn't matter. 2 registers with the shortest run come from the trunk further out and they read 66F.
What could be causing such a drastic increase in temps from one to another?