Maybe the form of those two horses provided no clue and therefore the punter/handicapper could not possibly factor the reason into his/her calculations.

Maybe only the trainer/stable was aware of the reason - maybe each horse had responded well to a change of training technique, had been primed for this specific race. In fact, there could be one or several specific reasons that the punter could not possibly be privy to?

If Mark Read, Sportingbet, successful handicappers, etc, had an opening price similar to yours or even rated them softer than you did, then "that indicator you are missing" might also be a factor missing to even the experts and everyone not "in the know"?