Have just tripped over a little CSS issue whereby the left had side border is becoming double thickness when the first column is hidden.
To replicate the problem run the
Grid / Column menu Demo
and hide the first column.
The results can been seen in the attached screenshot.
Digging into the CSS I can see you are removing the border on the first child
but when this column is hidden the subsequent column will still retain the left border resulting in the 2px border-left.