I believe they are still using Havok to make the game, just a more advanced version of it. I think this question could be Partially answered by an understanding of how the newer/current Havok engine has improved over the older versions that were used in Oblivion and Fo3, but most of it is in the modelling.
I see this effect every single day as I place clutter items in the GECK, the Havok engine always enforces the collision boundary for items that have collision, and usually those collision meshes are slightly larger and more generic than the object (like books, for example). What looks like a floating book to us is really a book with a slightly larger (invisible) collision mesh that Havok is reacting to - so I don't even know that it's fair to blame Havok for this effect. It's more to do with the 3D modelling of the collision mesh for objects in the game.
Problem is that the collision meshes I've worked with are almost always simpler than the original mesh (for performance, its alot easier to calculate where a door is by considering a perfect rectangle versus trying to figure out the seams, decorations and door trim). Even with a perfect mesh we still get _a little_ floating, but its almost too small to notice.
Perhaps a campaign for "Complex Collision Meshes Please!" would achieve the desired effect?