I have noted this with electronic scales, which are often out of the same small number of factories but re badged with minor flossies. AS has been suggested earlier in your thread a standard "calibration check weight set " available for less than $20NZ may remove the puzzlement.
Here is a link to a set i have ........... https://tinyurl.com/y42ufgtp
Failing that, and excluding drafty winds, have you thought of emailing Lyman and asking them why? a link to their "customer service" contact us email.
https://tinyurl.com/y6l9wsqv
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