ELI5: Don't understand these statistics

I'm an FCAS but don't understand how these percentiles can all be true for the same person (from a kid's medical check-up):

Height: 79th

Weight: 81st

BMI: 67th.

Since BMI is a function of weight/height, I thought someone in the same percentile for both would be roughly 50th percentile, so someone with the height/weight above would be low 50's.

Maybe it's because there is a big grouping around the middle, so the variance between 81 and 79 is the same as 67 and 50?

I'm assuming a bell curve -ish. I realize kids can't be 0 for height or weight but could be double the average.

ELI5