I have occasionally encountered similar, not identical problems, usually with the last row exhibiting semi-bold. We had one case in which this effect was cleared by changing the table's text wrapping to Around and then back to None -- go figure. At other times, I've had no luck removing the appearance. MY experience is that it comes in with tables pasted from other sources, even after all styles are cleared. Word tables can occasionally cling to hidden styles like a barnacle. I wish I could offer a definitive solution.