Ok, so percentiles are based on the average of the weights of babies. So as babies in general get bigger where a certain
weight of baby is changes. So, say a 7 pound baby was in the 8th percentile a few generations ago, but now because of better prenatal care they are in the 4th percentile (I made these numbers up). Also, since these numbers are based on birth weights of actual babies, by default someone has to be at the bottom.