There are two issues with thermometers.  Due to manufacturing tolerances even the same model of thermometers from the same manufacturing run can read different temperatures, either high or low.  That’s why you need to calibrate them.  The next time you are at a big box store or a hardware store that sells thermometers, look at the ones on the shelf.   I’ve seen 9 degrees F difference in the high and low readings in that climate controlled area where you’d expect them to read the same thing.  These are the ones you hang up outside to see what the air temperature is.  
The other issue is how inconsistent are they?  Most of them are fairly consistent, once you calibrate them and make your adjustments you can pretty much depend on them.  But some may be off a degree or more from one time to another.  The type you hang outside to see air temperature are generally accurate to within one degree.  They may be off a small bit, but not more than one degree.  
I use a medical thermometer to check mine.  Those are supposed to be calibrated before they leave the factory.