There is very often confusion about the tons of a vessel. It probably goes back to the origins of the name Ton.
Ton from tonel, from barrel.
A ship's hold had a certain volume. This volume could hold so many barrels of water or wine or whatever. Now, barrels do not pack very tight. There is a lot of space between because the barrel has a an odd shape. This shape is so because it is the only way that strips of wood could be joined together to make a waterproof container. Quite an art actually.
So a ship was taxed not by it's dead-weight, but by the size of it's cargo hold, the measure being barrels.
Still in modern times this system is being used. Engine space and living quarters, fuel tanks etc. do not make part of the cargo space and therefore the tonnage of the ship.
So a ship with a dead-weight, that is the actual weight of the empty ship if put on a scale, of 1000 tons, can carry several thousand tons of cargo. It depends how it is built.
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