Everyone has hit on anything I can think of, except possibly this....not knowing what your range is like, this is just a guess. Is the lighting at your 30 yd. target way different? Or, if you have one target and you move up and back for the different distances, is the lighting different where your 30 yd. mark is? I have noticed myself having slight left/right differences sometimes when going immediately from one extreme to another as far as lighting goes.
This is a good one, let us know what you come up with!

NRA,UBP,BASS Member
New Stanton,PA