They measure every employee’s production every shift and apply pressure to those who are not on target.
I once worked in a business, in which the salesmen were ranked and measured in a similar manner.
The attitude of the owner/boss of the company, was that he wanted top performing salesmen, and that he wanted to train and mold them into top performers. He knew not all would be able to make the cut, but he had high standards for performance. Those who performed were rewarded, and for those who didn’t meet standards, attempts were made to train them to increase their performance.
If some salesmen were ranked at the bottom two or three months in a row, that salesman would be replaced. The attitude of the boss, was that he would rather cut his losses, and then recruit some new people who might be able to achieve at the higher levels he expected, instead of having to carry lower achieving people who would never be able to improve.
It sounds as if Amazon has some similar employee standards.
If they score everyone against “the average” you will find the best performers in the group with their ankles broken.
Managing against standard deviations from the mean is OK, but expecting everyone to be above an average is mathematically impossible.