Undercoating?
Dealers often offer undercoating/rustproofing packages that they push hard when you're buying a new car. Places like Ziebart used to offer this as well. You can also buy it yourself at auto parts stores and apply it any time. So if you're looking at one that has full undercoating, then one of these reasons would account for it.
One would be much better off just washing under their car every so often than dealing with that practically impossible to remove stuff.











