the borders are probably causing this, when you add a 1px border to a table and set the table to 100% width, the width will probably be 100%+2px..
try calculating the exact width in px by using the Pixel per Inch and the document template width height.
For example 120 PPI, 8.268 Inch width = 992,16 px (992px) If you change the PPI value the pixel width will change to.