In the traditional sense (Teletimer style timing) the fractions are set by the FIRST horse to cross the activated sensor line (they activate about 5 seconds before the first horse comes by in order that trash blowing by or birds do not set them off prematurely). Horses close by are then effected by that pace of race, since they are required to output a significant (if it is fast) amount of energy just to keep up.
Horses more than about 5 to 7 lenghts back are not MAKING nor PROMPTING that pace. They have NOTHING TO DO WITH EITHER SETTING IT OR SPEEDING IT UP. We must then HOPE that the beaten lengths given as well as that altered fractional difference are adequately reported to evaluate ANYTHING about their contributions to today's pace of race. Also, one has to project that they MOVE and exactly the right time, the pace up front stays as projected, AND their is a path available for them to get up there.
Another very very important thing: EXERTION. The longer physical exertion is maintained, the greater the oxygen debt.
http://www.teachpe.com/anatomy/oxygen_debt.php
THE longer the time in oxygen debt or the great the early stresses on exertion, builds the greatest LATE energy demands on the individual be it human or animal.
So in a normal lively paced race, we are asking the off pace horse to INCREASE its anaerobic physiology exactly at the time that it is the most difficult (when early exertion has drained oxygen reserves) NO wonder earlier horses win more often: IT IS PHYSIOLOGICALLY easier. Remember Huey Mahl: for each unit of energy lost at the second call it takes TWO units to EQUAL it late, and that is just to TIE. At many of the bull rings I play that ratio has increased(especially in the cooler temperatures and the off going) to 1:3 or for short times 1:4.
We ALL like the Aldebaran's, Zenyatta's and Silky Sullivan's of this world but just remember, if these types are NOT head and shoulders above the rest, they just don't win their fair share HISTORICALLY.
Look at all the match races: One goes out and the other chases the whole way around: Zev/Papyrus, Seabiscuit/War Admiral, Swaps/Nashua. Convenience/Typecast, Chris Everet/Miss Musket, Alsab/Whirlaway.
Matches within a race (Affirmed/Alydar, Easy Goer/Sunday Silence etc). Speed goes out then the closer looks okay but just never quite gets up.
We deal almost directly with the horses on, or near the pace of the race. Why guess on the others?
Unless the match up SHOWS the pace of race that the speed ball contended against, is the VERY SIMILAR pace of race that the closer contended against, the latter should be downplayed substantially. That is the physiologically LEAST able time to come up with the extra in any part of the contest. Using a pace of race the is SLOWER early (for the line choice on the closer) often projects a greater 3r fraction velocity because this animal was NOT taxed early and we get a false projection of late moving, BUT FROM A SLOWER pace of race. Be very careful to project on similar paces of races.