My experience leads me to assume that the thermometer was mismarked. My high school chemistry teacher drilled into us that the thermometers we had were all precise, but of varying accuracy. A thermometer might say that water boils at 99.5 C, but if it did, it would also say that it froze at -0.5 C. Again, there are conditions that actually change the temperature at which water boils, so it's possible you were at a lower atmospheric pressure or that the water was contaminated. But, given that we have a grand total of one data point, I can't narrow it down to a single answer.