I think they also favor the newest machine sometimes. Kinda like the truck that wins Truck of the Year is always whatever is the new model.

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Andy Bassham *(1999 Arctic Cat 500 4x4, 1989 Honda 300)*

 

I'm not a Yamaha fan myself, but I have ridden the 99 and 00 models, and without question the 2000 is a better machine than its predecessor. I found a definate improvement in performance and styling. Unfortunately, I have only ridden the 450ES once, and therefore can't make a judgement call.
I have read that article SwampMonster, and I don't think it really matters whether the pilots are heavy or lightweight, just as long as they are equal for fair data results.