When Almanzo runs into Brenda Sue for the very first time, he gives her a perfectly normal "Hello, how are you doing?" and then goes on to say that he hasn't seen her in months. It was clear that Brenda Sue was an unexpected out-of-town visitor in this episode. If an old friend walked up to you out of the blue after all that time, wouldn't you be surprised to see them? The way Almanzo greeted her, you would have thought that he saw her every day of his life.