All eyes are unique so accuracy might differ for everyone and there could be a natural offset. Eye tracking will however never be pixel precise accurate because of how the human eye works. The human eye doesn't focus on single pixels, instead, our gaze point lays within a bigger area. We often say the accuracy will be the size of an averagely sized thumbnail.
Accuracy is deﬁned as the average difference between the real stimuli position and the measured gaze position. Precision is deﬁned as the ability of the eye tracker to reliably reproduce the same gaze point measurement, i.e. it measures the variation. See examples below, but please note that the dots are just a visual representation and don't reflect the performance of our eye trackers.
Explained in a more casual way, accuracy is when your gaze on average hits the target area and precision is when it can find your gaze point focus to a small spot.
Please note that eye tracking can be implemented differently in every game and software so results might vary.
Good accuracy and good precision
Good accuracy and poor precision
Poor accuracy and good precision
Poor accuracy and poor precision