I have always understood that Colonel F was concerned that his own behaviour might have given the wrong impression, and being an honest, gentlmanly chap, did not want to lead the lady astray. Perhaps Lizzy has shown what he thinks are symptoms of partiality, or, at least, expectation.

I would have thought JA would have mentioned such incidents, but she doesn't tell us which incidents Darcy based his mistaken opinions on. Maybe showing apparant favouratism even where she doesn't feel it is a side of Lizzy's character that she (even with her pretty good self-knowledge) is just unaware of, which is why JA doesn't point it out.