The Mechaber brings down from the Rosh the halacha that if someone says at the end of a bread meal “lets bentch,” he can not continue to eat or drink without first making a new hamotzei. (Shulchan Aruch 179:1) By saying “lets bentch” he is showing he no longer wants to eat, and anything eaten after that is like a new meal.If however, a guest at a meal says “lets bentch,” and the host informs the guest that there will be more food served, then the guest can continue to eat and drink without first making a new bracha. The reason is that the guest was mistaken when he thought the meal was over when in fact there was going to be more food served. Had he known that there would be more food served he never would have said “lets bentch.” (Shulchan Aruch 179:2)
The Aruch Hashulchan (179:6) takes it one step further. He writes that even when a host says “lets bentch,” if his wife says that she has more food to serve he is treated as a guest and he may continue to eat and drink without making a new bracha. When he said “lets bentch” it was the exact same mistake the guest made as he did not know that his wife had more food to serve.