About MS Excel 2010: why the border of the column with the Styles "Format as Table" & "Conditional Formatting" can not be arranged as a straight line?

Hi,

I am using MS Excel 2010 to design a report with "Format as Table" & "Conditional Formatting" (that are located in the Styles group on the Home tab).

I apply "Table Style Light 2" to the dataset first, then set up a rule (e.g. if the sales in week 32 > that in week 31, then the cell background color is red; otherwise, green) with "Conditional Formatting" to change the cell background color.

I get a problem with the border that can not be arranged as a straight line. For example, please check the attached image.

How to deal with the issue? Is this a bug for MS Excel?

Thank you in advance.