last lay price after race finish.

last lay figure after the end of a race is for me on so many occasions, not the true picture. time and time again the figure is lower than a lay l have placed. Why is my lay at say 1.3 not taken up when the final figure is say 1.12. ? This final Betfair lay is whatever
betfair decide to make it..How can this computerised figure skip my lay. You cannot trust getting a lay taken.