Digital thermometers can be tweaked a bit intentionally to display readings that are slightly lower or higher. For example, you can hold it very tightly in your hand to make it read higher than body temperature because the palm of the hand can get very hot and sweaty without air. \
Taking a drink of something hot or cold and then putting the thermometer in your mouth will also skew the reading. Even wearing a hat can affect the accuracy of the reading, sometimes. \