Hi Ray,
I am not a Perthshire specialist but I have seen some small variances on Pershire weights before. I was told in one instance, this had been a specific demand of the original buyer (the weight was unfacetted even though it should have been - it was signed, numbered and it had the original box and certificate - so I believed the reputable dealer who told me this). Since then, I have seen other examples of such discrepancies.
I am sure that Perthshire specialists will confirm (or not) my explanation though.
SophieB