Miss Bates gives us clues about those connected to Highbury.

Emma is bright enough to pick up on those clues but she allows preconceptions and prejudice to interfere with her judgement.

For example, Mr. Dixon saved the life of Miss Fairfax. I cannot imagine anyone in the sailing party to stand aside and allow one of the party to be knocked overboard. Yet Emma misses the obvious and draws a more romantic conclusion, and then looks for other facts to support this conclusion. If another gentleman happened to be in the way of preventing Miss Fairfax from falling overboard, Emma may have drawn the same romantic conclusion except by that other gentleman.