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+/- 30% would not be unexpected on commercial fasteners such as Grade 12.9 Cap Heads.
If you measure preload it a load cell and a carefully designed fixture the results are even more frightening.
You can reduce the scatter by 'burnishing' the individual nut/bolt prior to final torque tightening but with stretch as the ruling measurement there should be no need to carr out this operation.
The only question I have is why you have no measurements below 40 lbsft? This would be statistically unusual.
Have you skewed your results by always tightening to the minimum suggested torque ?
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