If you're beating the Betfair SP you are +EV. That is an indisputable fact.
Theres more to ev than the price of the odds
How do you calculate the things that can affect the outcome ?
If slim chance that day was just getting over an infection , unfit , trainer cheating or whatever you would have lost and not even know why
how can you calculate your bets over a long period of time when every situation is different and you dont have complete information.