Excel object in word document; some cell borders are too dark
 

I'm trying to work with an Excel table as an Excel object inside a Word
document. The problem I'm having is that some of the cell borders are darker
than others, despite the fact that I set them all at the same width. When I
open the table to correct the borders, they appear to be all set at same
width as well.

Can anyone tell me what cause this problem and how to make all the cell
borders behave?