Couple of possibilities, temp sensor is off which can be verified with a thermometer or battery heating/placement/configuration allows it to dissipate heat into the 3 more than the Y. I think the bigger question is what temp is it inside the garage? I know my Y pretty much mimics the temp inside the garage, Just looked at it now and the Y says 15c in the car and the garage thermostat says 16c, I take the difference as thermometer accuracy.