Readings should always be taken at 25C (77F). Unless a temperature is given, pH numbers are always assumed to be at 25C. If you take the reading hot, it will be a little lower.
Hot solutions are more acidic than room temperature solutions. ATC does not compensate for that.
ATC compensates for errors from the probe. Its designed for 25C, so the computer applies a little fudge factor when it isnt. Supposedly this is way better than twisting a little knob, like on the MW101 to set the sample temp. Guess which one I have.
If the sample is not at 25C, its actual pH will be different. Plus they say hot readings shorten the life of the electrode.