So it turns out that home oven manufacturers don't care too much about precision when it comes to oven temperature (and it doesn't matter usually).

A high end home oven will easily swing between +- 25 degrees of the temperature that you have dialed in, and that's if it's well calibrated.

If he didn't dial in "340" quite right, and his oven was average, I can easily see it hitting temperatures of 412+, and that could also account for his varied results.