The upper end is based on human body temperature, not the hottest days. IIRC they were either measuring someone with a slight fever or the thermometer was not quite as accurate as we have now. And 0 is not remotely the coldest where I'm from, that's a pretty typical temperature in January for us. We routinely see -30C here (-22F) and a few times each winter it will hit -40 (same either scale).