[f. beam n.1 + -age.]
A deduction for loss of weight by evaporation in cooling, made in weighing the dressed carcass of a beast.
1902 in Webster Suppl.

 OED2

In other words, referring to the effect of a weight variation as recorded on a beam (early form of balance or scales).

The later uses you found were extrapolating from ‘a beam of light’.