Curve fitting rule ??

Curve fitting is essentially the bell curve distribution where the majority of people being considered in the curve will fall in a central normalised zone ( think bands B /C) along with a few outliers across both ends ( A and D) . This is mostly used to standardise the band rationalisation process and the grading itself.

Lots of big IT companies follow this distribution and curve fitting is essentially telling you that your individual performance was near a certain part of the curve ( could be central or an outlier ) which is why you have been given a specific band.