On the rollers you are only moving the rollers while out on the road you are moving the vehicle and sometimes it's going to take anlot longer to get up the rev range out on the road.

Also, on the road you will get more issues with feuling as on the rollers a poor fuel set up isn't going to be as obvious as it is on a top speed run where most cossies suffer terminal issues

If I was on the computer I'd give you a long shpeil but I'm not