For consumer products such as fresh meat and cheese, the variable relationship between base units (such as pieces, boxes, or pallets) and alternative units of measurement (such as kilograms or pounds) means that fixed conversion factors cannot be used.

see first web reference
The concept of "catch weight" was developed because many seafood items are not sized uniformly like a can of tuna. One particular group or 4 Dungeness crabs may weigh 6.9 pounds while another group may weigh only 6.1 pounds.
At a traditional seafood market, shoppers are accustomed to having their orders weighed at the counter, and paying for exactly what was weighed

see second web referrence below