It isn't at all strange. It's what happens.
My waist didn't thin down--everything else expanded around it. I didn't really concern myself too much with weight pre-puberty (it hit me around elevenish, so before that I was pretty much a kid) so I didn't measure my waist, but I know that by fourteen or so, I was about a 38-26-37ish. I remember because I thought that was "fat."
I would LOVE to be that "fat" again.
You should check with your doctor and see what kind of measurements he or she can take to give you a better idea of your actual fat percentage. No measurement you take can really help you much, unfortunately, because they're all geared to people who have finished growing.
Edited to add: I should also note that my "puppy fat" got stretched over an extra couple of inches of height, so my waist may have thinned a little. But it didn't thin much; most of it was just natural reshaping as I grew boobs and hips (of which I had plenty then and have even more now.
)