Also, doctors are much more interested in the relationship between the percentiles for weight and length than they are the numbers themselves. Healthy babies tend to land in somewhat similar percentiles for both metrics and are probably getting enough nutrition. This goes for those babies who don’t chart at all. Because it is impossible for researchers to determine what is the largest and smallest possible weight and length for babies, designers begin the charts at the fifth percentile and end them at the 95th percentile. This means 10% of babies are, like my son, unchartable.
Source: wiktionary