Sounds odd, particulary as they were well fancied runners, usually huge amounts on BF when you have short priced favs. Could it have been that the price moved as your bot submitted its bet? If the bot fired off a bet at $2.0, and as it did, the market price shortened, it might explain it? You would have managed to grab some of the $2.0 before the market moved, leaving the rest of your stake unmatched, and if the price didnt drift back out to $2.0, its not at the front of the prices to be layed.