I recently read that in some cases the shaft may still be fishtailing when it gets to the target. If it is you will have a false reading. That is the reason for shooting at a different distance.
I was trying to bare shaft some 340 , shafts once. After 2 hit the target at a hard and snapped, I quit and only tune with fletched arrows now.
Last year while I was practicing at the Classic, an older gentleman was watching me. He questioned my choice of carbon and asked if we could experiment with one. He trimmed the feathers down to the quill then I shot it. Grouped with the others. He stated the weight of the feathers plays a factor in tuning and stripping down to the quill leaves most of the weight.