If you have a thermister type thermometer then yes, they are inherently inaccurate. The thermisters themselves have to be programmed into the controller using a coefficient and slope unique to each thermister. And even when you do that you will find that they change over time.
I programmed my thermisters to give the correct reading at their target temp (165 for the HLT, 152 for the Herms, and 150 for the MT).
I have laboratory grade mercury thermometers, but I don't use them on brew day. They are accurate, but slow to respond. And if you drop one in the BK or MT you might as well call the haz-mat team to decontaminate your brew rig!
Charlie