I've read everywhere that you cannot mount a Shimano 11spd cassette on a 9/10spd freehub.
Just out of curiousity I took the 9000 Dura-Ace cassette from my new C24 wheels and mounted it on an old (circa 2005) Mavic Ksyrium SSL 9spd hub. No problem. The lock ring had plenty of threads to catch, in fact the last cog provided the same reveal amount for the freehub and axle nut as on the C24s which have the "true" 11spd hub.
My rough measurement of the Mavic freehub length is ~33.4mm, whereas the 9000 freehub is something like 36.7mm. Is there something weird with the Mavic freehub that lets the 9000 11spd cassette sit further inboard than maybe other freehubs do? The resulting cassette placement appears to be perfect. I didn't even need to re-adjust the 9000 derailleur.
What am I missing here?
See for yourself:
For reference: http://www.bikerumor.com/2012/05/01/...ech-breakdown/
DT Swiss 240 hub: