The previous episode goes into Talmudic style rigourous logic to justify my claim: this is the 2 minutes summary. A) You care enough that you actually feel bad. B) In a field where there aren't clear standards, no standardized scores or lap times, "good" means what most people are like, including their bad days and mistakes. C) Your thinking that you did bad, suggests that you are someone who in this area that you 'made a fool of yourself', in that area, you pay attention to what separates a good performance and a mediocre one. In summary: You giving it your best (A), in a field that you studied (C), makes it hard to justify believing that you are worse than the normal performer on a bad day. Since good performance is defined solely on what people are used to (B), given (A) & (C), people must be thinking that you were either good, or a good performer with a glitch.