IE7 has problems with rounding up it's subpixels. Something that is exactly 100% in webkit or firefox can end up being a couple of pixels larger in IE7, the result of these extra couple of pixels can end up pushing columns below each other. The best explaination http://ejohn.org/blog/sub-pixel-problems-in-css/
It looks as though bootstrap remove 0.05319148936 for each column in IE7 to deal with this issue.
if you use sass you can make use of this mixin
@mixin x-percent-width($percentage) {
width: $percentage;
*width: $percentage - 0.05319148936;
}