Think of the “hardened surface” of an object like a invisible coating of paint. The designer makes an object, and hardens it with the “hardened surface coating”. Then a scenery designer decides to use that object at a location. But the object (in this case a wharf) must be resized (made smaller), to resemble a concrete pier. The pier that you see is resized perfectly...but the invisible “hardened coating” does not resize. It is always at the original size. Therefore when you approach such an object, your aircraft will react to the original invisible hardened surface and seem to jump in the air if you approach the object.
What I have done for some of these is put red buoys at the limits of the hardened surface, (Bear Observatory), warning a float plane or boat not to approach closer than the buoys. Or, the designer can change out the object. Or we can treat just live with it as we live with a “unhardened” dock you can drive through. But now you know what is happening.