Taking SP at start of a race

Hi, I'm a relative newcomer to Betfair so apologies if this has been answered before or I'm just being stupid but I've recently placed a few lay bets on the horse racing where I've requested odds at a certain price below the current betfair price and then set it to take the SP if unmatched at the start of the race.  When the race is over (the price I requested didn't match so it took SP) and I've won the bet I've noticed that I didn't actually win the amount I had entered as my back price.   Is this because the whole amount wasn't matched when it took SP or is it because Betfair doesn't increase your liability in line with the new odds and just changes the amount you back to accommodate the price? Any help appreciated.