When someone evaluates another person’s performance from the perspective of how skilled (or unskilled) they, themselves are in a particular area. As a result, they would rate someone else’s skills at a higher level if they’re not good at it, and rate others’ skills at a lower level in areas they are more skilled at.
Research suggests that, because of this bias, over 50% of a rating can be attributed to the rater, not the performance of the ratee!