Question

why is the boiling point of the top water did not reach exactly 100° celsius

Answer 1

Because boiling point of water is not 100 degrees Celsius but it depends on atmospheric pressure. Liquid boils at temperature when partial pressure of liquid becomes equal to atmospheric pressure. ... At hill station, due to low atmospheric pressure water boils at less than 100 degrees Celsius.
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Answer 2

Pure water boils at 100 C only when the ambient pressure is 1 atm. If you were to take your pot of water up to the top of a tall mountain and start to boil it, your pot of water would boil at a temperature significantly less than 100 C, because the ambient pressure is no longer 1 atm. For example, at an elevation of 3000 m above sea level, the boiling point of water is reduced to about 90 C. Conversely, if one heats water in a closed environment where the pressure is greater than one atmosphere, boiling will not occur until the temperature exceeds 100 C. The boiling point of water can be changed also by dissolving another substance in it or mixing it with another substance. That is the principle behind adding anti-freeze to the water in a car radiator: not only is the boiling point of the mixture raised above 100 C, but the freezing point of the mixture is reduced below 0 C. On a hot day, the water in the radiator stays liquid above 100 C because the cooling system also is pressurized to a certain extent. If the radiator cap is removed suddenly, the pressure inside the cooling system drops at once to ambient, and the hot coolant can begin to vaporize.