It's the same principle for restricting the number of children on board according to spaces in the children's club, and the number of trained and certified staff on board, even though many of the children won't use the services. They have to be able to do so if necessary. The same reasoning applies, even though some wheelchair users won't need extra assistance, they have to assume that they all will. They can't assume the opposite.
Same with lifeboats. They have to carry spaces for maximum number of passengers (and crew) even though the ship may not be rammed to the gunnels, they have to assume and plan as if it is.