BMI and school in the 2000s

I got a letter in the mail today from the school district. It reports Leanna's height, weight, and BMI percentile. She's 43 inches, 39 pounds, which is a BMI percentile of 29. According to them 5-85 is a normal weight (Wow, that's a big range!). I was curious what percentile that puts her in and when I looked it up here: http://www.halls.md/body-mass-index/bmi.htm
it said she's in the 18th percentile for her age and height.

I think it's a good thing that schools are making parents aware of where their child falls in the weight range, but it's kind of strange. So much has changed even just since I went school.