Dead freight means the damage caused by the failure to furnish a full cargo in accordance with the contract, not merely that caused by the ship not being in fact fully loaded. In other words deadfreight accrues only when quantity stated in contract was not supplied, which signifies a breach of contract, and not when the vessel loaded cargo as per contract but less than her full capacity.
Per Best J in Hunter v Fry EngR 340; (1819) 2 B & A 421; 106 E.R. 420 at p.422:
The stipulation in the charter-party is not that the owner should receive, and the freighter put on board, a cargo equivalent to the tonnage described in the charter-party; but that the one should receive a full and complete cargo, not exceeding what the ship was capable of receiving with safety, and that the other should put such a cargo on board. Now it is clear, that the freighter would be entitled to recover damages against the owner, if the latter had refused to receive on board any thing less than what the ship could carry with safety; and it seems to me, that the defendant, who has covenanted to load a full and complete cargo, is equally liable in damages, for not having laden such a cargo as the ship could safely carry.