The left/right boxes width applies to the individual sideboxes, while the column width applies to the column as a whole. If you have no padding or margins in the columns and sideboxes, you can use the same dimensions. If you do have padding/margins, you should ideally set the widths to account for them (but you will generally get consistent results even if you don't).
If you have 2px padding in the columns and the widths are set the same, the columns will actually display 4px wider than the boxes which will be at the set width.

The cleanest thing would be to set column widths only and let the boxes automatically fill them neatly.


The sidebox header bg fix is what I told you in post 2.