[QUOTE=moeee]
The result of a race is dependent on the immediate circumstances.
QUOTE]

Exactly.

How can anyone work out true odds BEFORE a race when the result will be dependent on immediate circumstances that happen DURING the race, that we the punter will not know until AFTER the race?

A horse you work out as having true odds of say 10/1 that loses it's jockey during the race, has not got a 10/1 chance of winning, it has no chance of winning. So your 'true' odds would have been false and therefore not true odds at all.

True odds can never be more than an educated guess, if they weren't there would be no horse racing as nobody would take our bets unless we accepted unders [just like the games in a Casino]!