Brachos 49 – ברכות מט

The Gemara teaches that one who omitted Shabbos/Yom Tov from his Birchas Hamzon and has already begun Hatov V’hameitiv he has to recite Birchas Hamzon again.  Rashi explains that starting the bracha of Hatov V’hameitiv is equated to having completed Shemone Esrei and uprooting his feet (which requires one to return to the beginning of Shemone Esrei if he omitted something) since he concluded the first three brachos (which are biblical).  But haven’t we learned that if one is accustomed to adding ‘divrei tachnunim’ (prayers of supplication) at the end of Shemone Esrei, it is considered as though he is still in the middle of Shemone Esrei – and here too since he is still adding more to his Birchas Hamazon why don’t we consider him as though he is still in the middle?