There will always be some difference between individuals and the accuracy of a calibration. However, the rating function is an approximation that doesn't tell the full story. It's possible that a four star is fairly close to a five since the cut off is a hard limit. The rating is done like this (values are in degrees of visual angle):
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if (accuracy < 0.5) // perfect
return 5;
if (accuracy < 0.7) // moderate etc.
return 4;
if (accuracy < 1)
return 3;
if (accuracy < 1.5)
return 2;
return 1; // redo
So a 0.51 calibration would yield 4 stars but it's a still a good result
Try to check the accuracy with the mouse-redirect, is it constantly of any specific area of the screen?
The primary factors affecting the result is screen size and distance from the device (other than individual differences in pupil size, eye shape etc).