The rules for admeasurement of boats and ships are designed for cheating. Each nation aims to favor their own ships and penalize those of other lands. Each owner wants to reduce the tonnage as much as possible to avoid fees and charges. Every naval architect wants to satisfy the owners. Every nation wants to collect as much in duties and taxes as possible.
As a result a confusing and inaccurate system of admeasurement has grown, like a virus, with traditional, not written, exceptions and exemptions of great complication. Variations in conformation of the ships and boats create variances.
For example, sea-going vessels store cargo in the hold (as a general rule) while steamboats carry it on deck. According to most rules the deck is not suitable. Western rivers steamboats frequently had capacities of double their admeasured tonnage because the space above deck was not admeasured.
Today we have the phenomenon of containerization. Do the inspectors admeasure the ship only? Or do they admeasure the containers? If so, how?
You have opened a can of contrary and lively worms.