Wouldn’t they be averaging for all users, ie woman and children as well, prob guestimated that there are higher numbers of female / younger walkers by percentage hence lowering it to 60kg while MTB tend to have a higher proportion of big hairy chubbies increasing it to the 75kg as quoted.
EDIT – sorry higher proportion of well hung muscly types
Given the low % of both I’d hazard they are still well under (and I’ll add my extra 20KG into the equation)