CH-- not only do we keep track of how much mud is on each horse and jockey, we have a guy go out, scrape it off, put it in a separate pail for each horse, and weigh it. But only at the major tracks. You've never noticed that after a race?

I've made both the specific point (that horses behind the frontrunners can pick up dirt/mud which weighs something) and the general one (that figures are inherently limited in accuracy because the underlying information is not perfect) before. Other examples are:

Wind (speed and direction estimated by humans, usually before and after the race, because they are doing ground loss during it),

The effects of wind (there's a really big building there),

The effects of weight (which would vary depending on the weights of the horses, which we can't get),

Weight itself (see "Elliott Spitzer"),

Ground loss (paths estimated by humans, and invariably rounded off-- 2-3, rather than 1.75-2-2.3-2.9), and

"Beaten lengths" (see, "teletimer companies").

Believe it or not, there is a speed figure company that for many years claimed quarter point accuracy.

"Now I am ready to change the variant to fine-tune the numbers based on an analytical look at each horse's development-- but if I change one figure, I must equally change them all. If I want to change Holy Bull's provisional rating for a 1 1/4 mile race from 5 1/4 to the more likely 4, I must subtract 1 1/4 from every other horse -- including sprinters-- running on the dirt that day".
--- The Odds Must Be Crazy, by Len Ragozin, "with Len Friedman".

That might be the least true statement about making figures ever made.

Unrelated-- you will probably be surprised to learn that a guy who is in the running for best active professional horseplayer (and an occasional poster here) asked me yesterday why I go so easy on you.