Bhagwan, it can't always come to 18.5%. You might be confusing Market Percentage with the Commission and even Market Percentages vary.

TAB Win Market Percentages (at 14.5% commission) will vary from 117% to 119% depending on the effect of rounding, the total commission deducted including the rounding component will vary from 14.5% to say 19% if a $2.09 (rounding down to the $2.00 divi adds another 4.5%).
You cannot work out what the total commission is on the TAB's after rounding because we don't know the actual dollar amounts invested on each runner, but a guess is that rounding adds another 1.5% commission to win markets and more to place markets because of the lower dividends and greater affect of rounding.