This is a pretty common one that occurs whenever one did not quite hear what a person said and just wants to be nice...it stems from you not listenin to a person's sentence and assuming they said 'Have a nice day" or some shit...
McDonalds worker:"Your total is 13.42, pull through to window 2..."
Customer: *pulls around, hands money to cashier*
MW: "All right, enjoy your meal"
C: "You too..."
See, the worker doesn't have a meal...he can't enjoy his meal too. It makes no sense, and we all do it. I avoid it by just sayin 'aight' whenever I didn't hear you...