Cell shading appear outside table edge

I am creating a basic table with dark green shading in the first row,
and lighter green shading in the other rows.

When I print the Word doc to PDF (using PDF995), part of the shading
appears just outside the table, it looks like a slight extension of
the bottom border of the first row, but there is no border applied
anywhere in the table. This appears in the document too. It does not
appear if I print the document out to a normal printer (HP Laserjet).

Is there anyway of getting rid of this?

Thanks