Remember that she has already read his letter, which caused her to realize that she had misjudged him in many respects. The way I see it, when she arrives at Pemberley she's curious about who he really is. All of the wonderful commendation of Mrs. Reynolds serves to begin painting a picture of his true nature, to which she is receptive because of the letter, though still skeptical. Seeing the portrait gives her another perspective - it shows a more carefree Darcy from the days before his life got so complicated. But she's still not completely convinced - remember how surprised she is at his excellent behavior when they meet a few minutees later and over the next few days. But, when she sees how constant he is in that, she is further encouraged in her new-found good impressions. Even then, though, she has more to learn about his goodness over time before being sure in her new impression of him.