If it's a One Bedroom Villa which truly only SLEEPS 4, they will only book 4 guests on a cash reservation. This would include Saratoga, BoardWalk, Beach Club and Wilderness Lodge. DVC allows members the one extra occupant but that courtesy is not typically passed along to cash guests.
Resorts like Old Key West, Bay Lake Tower and Animal Kingdom which sleep 5 with the twin convertible chair should be allowed to book 5 guests on a cash basis as well as DVC points.
I think the policy has to do with expectations for the cash guest. Have you ever made a hotel reservation only to arrive and find that the room doesn't have enough sleeping capacity for your entire party? Hotels don't typically accept a reservation for 5 occupants for a room that has one King bed and one Queen sofabed. I guess they expect DVC members to be better aware of the limitations and prepared to adapt.