My understanding is that the battery mustn't get too cold so a heating element is automatically turned on during cold nights (you can't leave your iPhone or lap

My understanding is that the battery mustn't get too cold so a heating element is automatically turned on during cold nights (you can't leave your iPhone or laptop in your car during freezing weather either). This explains part of the loss in range. The second part is that the charge level is less accurate when the battery is cold, so only after driving for a bit you see your "real" range. Either way Tesla recommends you always plug in your car at night.