Diagnosis
The built-in sensor is being influenced by a local heat source or poor airflow. The ecobee itself generates a little heat, and if it's installed on a wall with hot pipes, in direct sunlight, or in a recessed 'thermostat pocket', the reading will be skewed.
How to Fix It
The main fix is to use a remote sensor. The ecobee Lite is compatible with ecobee SmartSensors. Place a sensor in the room where you want the most accurate reading and tell the ecobee to follow that sensor for your 'Home' comfort setting. This bypasses the thermostat's own sensor.
If That Didn't Work
You can try a manual calibration offset. From the thermostat, go to Settings > Installation Settings > Thresholds > Temperature Correction. You can adjust it +/- 5 degrees to match a known good thermometer. This is a band-aid, but it works.
Heads Up
Don't install the thermostat on an exterior wall or right above a heat register. The 'Temperature Correction' setting only affects the built-in sensor, not any remote SmartSensors.