Boiling point of water varies based on atmospheric pressure, water boils at a lower temperature high up in the mountains, for example, which could be why the estimated human body temperature is set as a standard, because it is consistent to reproduce as long as there are people around.

The freezing point of water is not affected by atmospheric pressure, however, it is affected by dissolved material in the water. Using a saturate solution of a salt would establish that consistency as well as lower the freezing point to create a bigger temperature range.