The default left and right margins for a table in MS Word are 0.08". However, the DocxFormatProvider uses 0 as a default value and overrides the padding when any of the top, bottom, right, or left is applied. For example:
<w:tblCellMar>
<w:top w:w="2880" w:type="dxa"/>
<w:bottom w:w="2880" w:type="dxa"/>
</w:tblCellMar>
after import/export results in
<w:tblCellMar>
<w:top w:w="2880" w:type="dxa"/>
<w:left w:w="0" w:type="dxa"/>
<w:right w:w="0" w:type="dxa"/>
<w:bottom w:w="2880" w:type="dxa"/>
</w:tblCellMar>