The '18's don't feel that crisp at the bottom of the rev range compared to the 21 (and 28 even more so) for some reason (it's not reflected in what the dyno says the bike is doing) so you probably end up using a lot more throttle just riding around normally without really realising it.

Also, how many people do the GPS wiresplice on the '21? Without that it'll never make the best power in the top gears and thus never use the fuel.

A lot depends how you ride too, Wayne and Steve aren't known for taking it easy and both have other very very fast roadbikes. You can bet their NSR's spend a lot of their life working damn hard.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread