Too many variables: Machining tolerances, the alloy the manufacturers use as a base reference versus your casting alloy, melt temperature, different throat and groove dimensions, etc. . .

I've yet to have a boolit that drops at the manufacturer's nominal weight, while boolit size's have varied by as much a .002" under nominal to .003" over.

Boolit casting is not as easy as we may like it to be, thus we learn to adapt and improvise.