Now thats news ! How the hell did that happen??!?!! Dev kits too powerful ? Seriously, thats like removing a major chunk of geometry !!! On one side we have Ryse and then this "next-gen" stuff.
The final version of Ryse has a lot of geometry removed as well.
Apparently this is normal for E3 to retail games. All ubisoft games have it, killzone2 had it, and Ryse and Forza have it as well.
In this case, the game was probably produced for not final hardware. As many rumours prior to release pointed out to a dual APU setup for the XB1, it's possible that the forza 5 E3 showing was targeting that. Think about it: excellent AA, volumetric fog, high quality motion blur, as you stated: a lot of extra geometry, better (realtime even) lighting, some textures as well. Shadows also.
What could be a better explanation? I really don't know.