Inaccurate Percentile Calculation

I dont know how theyre trying to calculate the percentiles by age, but its clearly not working. None of the percentiles in the app have matched up with either the hospitals or the pediatricians data for my son--by a long shot! Even without the doctors records telling me hes somewhere near the 75th percentile, its obvious that an 8 lb boy is NOT in the 30th percentile for weight for his age at 13 days old, as the app reports.

jtoxx about Growth: baby & child charts, v1.17