Milk protien discrepancy

I figured if anyone would know, someone here would. I was wondering what accounts for the differences in protein content between manufacturers, and in some cases between different volumes of milk?

For example one manufacturer says 8g/cup others say 9g or 10g. Some manufacturers have differences between whole milk, 2% and non-fat. Some even have differences between a 1 quart container and a 1 gallon container of the same type of milk (i.e.- 1qt 2% and 1 gallon 2%). It normally ranges from 8g/cup to 10g/cup and the variance is usually only 1g, but I was just wondering if anyone knew why?

thanks.

chance

I've noticed that also. I never did get a clear answer from anyone

Some milks are fortified with protein, generally skim milks.

Whole milk will have less protein per cup than skim milk because of the fat mass.

Other descrepancies are due to rounding.