"The average margin of a victory in a dirt route is nearly double that of a turf route. The difference in sprints is similar when distance is considered. Turf races are noticeably more contentious. This is vital information. With margins much tighter in turf races, trips become extremely important. Losing ground on turns, traffic trouble, and expending energy in a non-efficient manner can doom any horse in a grass race. The biggest take away from the above should be clear. A horse that loses a turf race by a defined number of lengths is not comparable to a horse that loses by the same number on dirt."
The above quote is from a great article by CJ about turf racing.