The upper centile is often used in statistics where you want to find useful results.
Under many cercumstances,
The top 10% have no friends, don't talk, use their cars and finish very early.
The middle 80% are the mainstream.
The bottom 10% are usually pissed, slow and don't want to be there.
You would normally take everybody and ignore the top and bottom 10% of employees so that they don't unfairly affect the 'normal' postie.
Then, you would take the mean average or most common normal of the remaining 80% of posties.
Royal mail have chosen to take a quite ridiculous stance that everyone must match up to those who are borderline rediculous.
e.g. the top 10% are millionares, the bottom 10% don't even claim job seekers and Royal Mail expects everyone else to live up to those on £60k per year!