More than likely design and materials of the thermometer. Flawed calibration. Different readings can occur from site taken, ie...forehead scanner, oral, rectal, ear.

It's probably a calibration issue. Use the newest one.

The type of thermometer can make a difference. Forehead scanners may give a different reading than tympanic, oral or rectal.

Hi!

Unfortunately, this is not uncommon to have different readings. But they should not be considerably different. Definitely within 1 degree. Make sure you are using in same location on the body. That can make a difference. Ultimately, if they are close, stick with one so you are able to at least see if temperature is going up or down.
Hope that helps.

Suzanne Freesemann

It is how well they work. One may be more accurate then the other. As long as they are pretty darn close its ok.