Originally Posted by
HillRider
Some 7 and early 8-speed freehub bodies had splines that ran all the way to the very end of the body and would not accept an 11T cog. Newer bodies were labeled "Hyperglide-C" (the "C" was for compact) and had the splines shortened about 1mm and would take an 11T cog. A few minutes with a Dremel could convert the early bodies to the C-configuration if you wish.
Sheldon Brown's web site has a pretty thorough explanation of this.
But why did the 11t cog not work on the 8 speed cassette but it did work on the 10 speed cassette? I'm pretty sure mine is the Hyperdrive version, but would have to look again to be sure.