Thermometers, like all measuring devices, need to be calibrated if you have need for accuracy. You need to check the thermometer against known temperatures, which isn't easy, except for 0 and 100C, but those are out of range for aquarium thermometers. Almost any thermometer will show you the changes in temperature with some accuracy - whether the temperature increased by 2C or 4C, for example.
Unlike many measuring devices, a glass thermometer, with the scale etched on the glass, can be manufactured to be very accurate, and since nothing can change that affects the reading, they should always be as accurate as they started out. But, glass thermometers with scales attached to the glass tube can't be trusted to ever be accurate.
Is it really necessary to know our tank water temperature accurately?