Diagnosis
Lid-mounted thermometers are notoriously inaccurate. They measure air temperature at the top of the dome, not cooking temperature at the grate.
How to Fix It
Do not rely on the lid thermometer for precise cooking. Use a reliable, dual-probe digital thermometer. Place one probe at grate level in the cooking zone you are using. This is your true cooking temperature. Consider the lid gauge only a rough indicator.
If That Didn't Work
You can test the lid thermometer's accuracy. Boil water. Hold the thermometer's stem in the steam (not touching water). It should read 212°F at sea level. If it's consistently off by a fixed amount, mentally adjust for that error.
Heads Up
Never try to recalibrate or adjust the built-in lid thermometer. It is typically not designed for user service. Its primary function is to indicate a general pre-heat range.