zzr250 niche

Went for a big ride yesterday that incorporated the Eildon-Jameson rd. I have a 2000 zzr250 and was with a couple of mates on their 600s (r6 and 675). These blokes are frequently impossible to stick with until the road becomes a bit dirty.
Once there is a little detritus on the road the zzr suddenly becomes the fastest. Why? Has anyone else experienced this bizarre phenomenon?
both my mate's claim to be losing the front and generally having a bad time whereas the little sports tourer just wobbles a bit and ploughs through.