They have a very complex way of working it all out, I believe they also do quotas and time frames, ie every month they take on 2/3/4% of all they insure belonging to top group cars, then every quarter, etc........so if you enquire about a top group car on the wrong day/week/month........you get the expensive quote....or so I'm told.
You do have to spend a few hours on this but well worth it
P