I looked this up once, jane. They don't use the same criteria for children. I seem to remember it's more to do with averages.
My son was 10 and half pounds when he was born and was above the 98th centile for weight. I was nagged all the time by health visitors about his weight, until I got a bit shirty. By the time he started primary school, he was taller than any of the other children and still heavier, but by then height was taken into account and he became 'normal'.
He's 6'4 now and of average build. I try to keep a non-obtrusive eye on him, because he has a higher than average risk of developing diabetes, given his family history.