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There is.
Although there's no denying the skill differential in descending, there is a great deal more potential for just bad luck when you're descending a mountain at 40mph+ on bicycle, than if you are climbing.
I still think fair enough that Nibali attacked at that point, but I can't help but think that it's a slightly less satisfying way for Kruijkwijk to lose pink. But, cycling's not fair so sod it.
There is no difference between dropping someone racing up hill (and they crack) and dropping someone down hill (because they crash).