It's impossible to accurately rate horses. I've seen numerous instances where Don Scott and Mark Read both claim(ed) to have "true ratings" yet differ markedly in their prices.

Maybe the missing indicator is luck or even opinion. For example, if a horse meets interference during a race then different handicappers may have a different opinion about the extent of the interference. Or even differing opinions of a horse's expected improvement. There are so many other factors that can be considered.