I had a couple of pounds of butter in the fridge, holiday season (and we are having lobster on Christmas, and always use a lot at this time). One was a brand name we all would know and one was the store brand.
Both weighed just a tad less than a lb, with paper on.
Was a chemist for many years and I do know that the calibration of instruments can, and will, vary. And therefore if exact weights are important, must be checked regularly. So we used to have a series of standardized weights that we would use to test and, when necessary, adjust the scales periodically (sometimes we were dealing in the nanogram range and accuracy, and precision, were very important).
No such luxury on the battery run thing we have at home, or I will guess those which most home folks have accessible. Nor do we need it.
Just one possible reason for variance.
And I would guess the butter has to weigh exactly one pound when it leaves the factory. If the product loses water over time I have no idea.
Just one thought.