I find it hard to believe that they would preprint a wrong rate in case you came early. It sounds like they made a mistake, to me.

This was then coupled with them not admitting the mistake. Their first reaction was to assume that they didn't make a mistake, and try to explain the higher rate. (Chances are it wasn't the person you talked with that made the mistake.) It seems very unprofessional to me that they would accuse you of gaming the system because a flight was 30 minutes early.