Goose - the offset is the measurement @ mash temperature (which varies if you step mash or decoct) vs pH at room temp (really 68 f for most meters). My garage in summer is never under 90 f - but I trust my device’s ATC and device chart offset for temp to determine my “room temp” mash pH.
That is not what ATC does. ATC does not correct pH, it corrects the slope of the units internal voltage potential equation such that the pH you read at a given temperature (within it's range) is the real pH, but it does not correct pH's such that a pH taken at one temperature is the same as a pH taken at a different temperature. Without ATC you must calibrate at the temperature of the reading whereby to get a correct reading at any temperature which differs from your calibration temperature, but ATC allows for some degree of temperature variance between the calibrant and the sample.
The thing that is hard to understand is that if you read (say) 5.50 pH at 90 F., and then you read (say) 5.56 pH at 68 F. (for the very same sample) both are de-Facto correct pH's (thanks to ATC) because more H+ (acid) ions are dissociated at 90 degrees than at 68 degrees (which is to say that dissociation pKa and Ka values vary with temperature, and is also to say that hot liquids are inherently more acidic than cold liquids).
What I believe that you are hoping for (along with millions of others) is that at both 90 degrees F. and 68 degrees F. the presence of ATC will force the display to read 5.56 pH for the same sample. That would be forcing the pH meter to lie. They are made to not lie. And ATC improves their ability to not lie, as opposed to their ability to lie to your satisfaction.