Because

The FPLA relates to the net quantity of contents information on packages, goods, or commodities that are sold on the basis of weight or measure (i.e., it does not apply to such products as electronic or industrial equipment that have contents sold by the quantity of their contents and appliances

https://www.nist.gov/standardsgov/compliance-faqs-packaging-and-labeling-us#2

Not trying to be snarkey but it literally was a 2 second search. There are laws against this, and it used to be standard practice to put a small amount more than actual weight (volume) listed bit now it seems they stopped giving a shit and dare you to question or sue them into compliance.