susy framework generating incorrect % widths

[suncity65]

First time using the susy compass plugin, I've created a fluid 12 col grid and have added two columns to this as shown in snipper below. My problem however is that the % seem to be incorrect, my parent container is 100%, my main col 9,12 cols and the secondary col 3,12. I guess these should result in main col = 75% and sec col = 25% but instead my results are main col = 74.57627% and sec col = 23.72881% ?CSS/SCSS:\[code\] $total-columns: 12; $column-width: 4em; $gutter-width: 1em; $grid-padding: 0; $show-grid-backgrounds : true; $container-style: fluid; .l-main-news{ width: columns(9,12); float: left; > div{ width: columns(4.5,9); float: left; }}.l-side-news{ width: columns(3,12); float:right;}\[/code\]