Earlier this week, we were in two different hybrids.   Same fertilizer, weed control, fungicide pass, all bone dry 13-14% moisture.   Basically just divided by a dirt road.   One of them was consistently making 63,000 pound truck loads.   In the other one, I don't think I ever fit 57,000 pounds on a truck.   Same grain cart, same hopper bottoms.  Had it been a test plot done only with yield monitor for measurement, I'm sure it would have been obvious there was a difference between Hybrid A and Hybrid B, but I doubt it would have been truly accurate with that much physical difference in the kernel depth and weight of the corn no matter how well the calibration was done earlier in a Hybrid C.  

Yield monitor GPS is valuable for spatial information, grain cart scales for knowing what actually goes out the driveway.