Too many parents consider their children as their own status symbols. "See, I'm totally awesome, because I made THAT!" So if you say something that indicates your child is doing better than theirs in some way, they perceive it as an overt statement that you're better than them (because they assume you do the same over-identifying with your child that they do). This leads them to a defensive posture, as they attempt to protect their own silly insecurities.

So, I'd explain this to my own teenager, and my message would be, "Don't let that parent's insecurities feed your own."