that Mr Knightley has been speaking more kindly to Harriet in part because he is sorry for having thought and spoken about her (though never to her) rather unkindly in the past? He has gone from seeing her just as an unsuitable friend for Emma and as the silly girl who rejected and hurt Robert Martin, who Mr Knightley likes and values, to acknowledging "Harriet Smith has some first-rate qualities, which Mrs. Elton is totally without. An unpretending, single-minded, artless girl -- infinitely to be preferred by any man of sense and taste to such a woman as Mrs. Elton. I found Harriet more conversable than I expected."
(Chapter 38). (Though it is rather a back-handed compliment to say she is preferable to Mrs Elton).