Did the carrying capacity include barding, feed, and stable hands as well as the horses?
Short answerThat is the proposed explanation: the horses themselves leave a substantial allowance for equipment and support.
NotesSixteen horses do not by themselves explain the cited seventy-ton capacity. A viewer suggests that barding, feed, and stable hands account for part of the difference. The response agrees that considerable additional material must be involved, using a heavy Shire horse of about 1.1 tons as a comparison. This is an interpretation of the capacity figure, rather than a detailed loading plan.