It's for the old Si / SiSL arms that have a slightly different profile. Using a 104mm axle with the new Si Hollowgram arms might give you chainline and chainstay clearance problems.
Thanks for clearing that up!
@hypercolour started
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It's for the old Si / SiSL arms that have a slightly different profile. Using a 104mm axle with the new Si Hollowgram arms might give you chainline and chainstay clearance problems.