Fetal percentiles

Samiantha101

So I had an u/s yesterday. According to the technician, my baby is weighing in at 4lbs 8 oz. I, of course, know these numbers aren't exact. She stated that it put my baby in the 40th percentile. When I looked online at the charts, her estimated weight only put her in the 24th fundal percentiles. So I have to ask, how are these calculations calculated? I mean, I get the idea, however why the discrepancies between the two?