What is accepted tolerance for new aftermarket valve springs?

That spread in rate is not all that huge; that is a total spread of 8 lbs, which is +/-1% from the mid range value. I would not call that dismal; I'd call it normal manufacturing tolerance.

And some potential difference will be in the measurements; just because the gauge has a nice digital readout does not mean anything about the accuracy of the equipment's calibration that you are using. If that equipment is calibrated 2% differently that the Comp equipment, then that would show the difference in you mid-range rate value vs the Comp number.

Then there are minute variations in the spring wire that are going to effect the rate too. It may seem hard to believe, but just a .001" change in the spring wire diameter will make a >2% change in spring rate for spring wire of this diameter range.