Anticipated Salutation?

Does anybody else ever answer a greeting prematurely? I work at a store and always say "Hi, how are you tonight"? The usual response is "Fine, and how are you"? I , under most circumstances then reply "Pretty good", "Fine thanks" or  "Just fine". Every once is a while someone will just say "Fine" and leave it at that. I admit I have been known from time to time to anticipate that they would ask it back and then they don't and then I fine myself saying "Pretty good" only to realize that they didn't ask me how I was doing and then I just stand there feeling like an idiot. I was just wondering if anybody else ever does that?