If you have a measurement tape with 1000 irregular markings with no system to them. You can use it as a measure of a persons height by reading off which marking

If you have a measurement tape with 1000 irregular markings with no system to them. You can use it as a measure of a persons height by reading off which marking a large group of people end up closed to. But reporting those marks make little sense, since they are irregular. So instead you report that a person is at the marking for the %percentile of the revenge group. It’s easy to see how such a system can be used to measure a decrease or increase of height over time, even though your normalizing your reporting and even though the markings are irregular.