The cells of the grid might have borders and might paddings which increase the total width of the columns. Normally the cells have 7px padding on both sides and 1px border which increases the total width of the column by 16px. You should take this into account when calculating the column widths. Or you should apply different styles that remove the borders and paddings of the cells.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!