For some unknown reason, I took the rotors off the motors and then twisted each armature one by one. Two of them felt the same as far as the amount of force it took to twist, and the other two took considerably more effort. The magnetic attraction of the field on the armatures of two of the motors is much stronger. My question is, wouldn't that make a big difference in the way the motors run when under power, and if it does, wouldn't that make the Phantom harder to control? Or, are the ESCs able to compensate for any differences in the power level of each motor? The harder to turn motors are the ones with the black dot props. The Phantom seems to fly ok, so maybe I'm being overly curious.

In theory it wouldn't make it harder to control, at least in GPS or ATTI mode. The IMU, GPS and compass provide input to the software and hardware that controls the motors in order to fly the Phantom. As long as the motors aren't physically impeded from rotating by debris, bad bearings or contact with the shells arms the flight software should be able to adjust the motors to provide the proper attitude.

If the difference you feel really is due to the armature and the magnet field I personally wouldn't worry about it.