Well some laps the drivers will be doing all similar times, but then randomly out of nowhere they will go like 3seconds faster one lap. It was a huge problem at this race:
http://igpmanager.com/play/?url=race-result/7#race If you look at the top two cars (ID:8312, 21664) they are just so much faster than anybody, but on the same tyres. (This was also a wet race, which seems to be where i have found most of the problems before, it was said to be light rain, but full wets were around 2seconds faster per lap which seemed odd)
It could be that the drivers are of such low quality, but it just seems strange to me.