I think it works like this:
The first person off the dock set's the scoring "curve". If he totally kills it, stand-up run, he still gets scored a 5 out of 10. If the next guy rides a little better, he'll get a 6 or 7, and if anyone rides worse then they'll get a 4 or 5. In Shane's case, he was probably last off the dock and kicked everyone's butt.