Originally Posted by
Metaluna
That's because the HG-800 (and the new 105 variety) is basically an 11 speed MTB cassette, and there is no special 11 speed freehub for shimano MTB wheels.
And to demystify why, the inboard sprocket is cantilevered over the spider on mountain bikes because the inner sprocket is large enough to clear the spokes.
-mr. bill