My first suspect would be the thermometer. What type are you using, digital, dial, or tube type? For the wort to have a roiling boil like you said I would think you would have had to reached 212F at least. I wouldnt really worry about it, as long as you got a boil your wort should be safe. Maybe try another thermometer as a comparison, put both in a pan of water and bring to a boil. Then compare the readings between the two. Hope this helps.