I can tell you that bows do vary considerably in measured weight on my scale from one bowyer to another, and even from one bow to the next with some. I see a lot of different bows, and weigh them all on the same scale. Even if my scale is not correct, I would expect to get consistently high or low readings.
The majority of the bows I weigh actually come in less than they are marked. Some by a pound or two, others by more. Occasionally, but less frequently, my scale shows the same or slightly higher than what is marked. I did have one older Bear recurve that was marked at 45# and my scale read 54#. More commonly, a bow marked at lets say 55# will weigh in at 52 or 53#. The inconsistency from one bow to another is suprising.
I did use a bag that was weighed at the local feed store to calibrate my scale. Maybe my methodology is wrong by a bit, but it does weigh a know amount correctly.