That's because this Template Monster template's stylesheet explicitly sets the center column width to be 350px (class m1) and 352px (class cont_center).

The template has clearly been designed as a three column layout, and coded that way. This is unnecessarily inflexible and reflects poor implementation, as does the ridiculous extent of the nested divs that will be a real pain to wade through for normally simple changes, and the use of CSS features that Internet Explorer doesn't recognize, which is why the site behaves differently in IE to how it behaves in other browsers.

I wish you luck.