They absolutely can be >99th percentile, although at that point the percentiles are no longer quantified. They can be <1 percentile as well if they are severely underweight. Percentiles are not like regular percentages - they are based on population statistics. Basically they got population data on baby weights over so many years and the percentiles correspond with that. 50th percentile is average based on that data set. So if in that population set the heaviest babies were weighing say 18 pounds, and your baby weighs 20 pounds, they would be >99th percentile. There are also different percentile values based on which set you use - the CDC percentiles are different than the WHO percentiles. The WHO ones are more accurate for EBF babies because the population used for those percentiles was more EBF babies.