Calibrating temperature probes

Background first: Two significant points exist for calibrating a
temperature probe or thermometer without comparing it to a known probe or
thermometer -- freezing and boiling points of water. For the freezing
point, one mixes up a slush of ice and water, the probe should read zero
degrees centigrade when immersed in the liquid. For the boiling point, stick
the probe in a pot of boiling water and it should read 100 degrees. Except,
of course, for the barometric pressure. We all know water boils at lower and
lower temperatures (?? 3 degrees per 1,000 feet??) as altitude is increased.

Question: Why doesn't the same pressure effect occur for the freezing
point?

Oh yeah, to make this an aviation related topic consider calibrating a
temperature probe or thermometer to use as a reference to check the OAT
gauge in your plane once in a while. You all do that now and then, don't
you?