I used the thermometer from my grill to measure the temperature in the oven; it read about 10 degrees lower than what I had the oven set at. (340 with the oven set at 350)

While I can't answer your oven question, if you want to check the calibration of your thermometer, it's really quite easy. Put your thermometer in a glass of ice water for 5 minutes. Check the reading. Adjust so it reads 32F. This was how we had to adjust when making pizza dough to ensure the water was at correct temperature. Thermometers are often wrong, and it helps to recalibrate it frequently.

Pam