Waist Not

I'm curious.  How can jeans claim to fit a certain inch waist when, in reality, they come a full eight inches shy of that natural waist line?  Do they use reflection to figure out where the pants would hit if they came anywhere close to the waist?  Who would've thought that jean designers (or at least the people that make the labels) would need their high school geometry?  Seems like they should just have hip measurements instead.  All jeans cover hips (so far as I know), but it's rare to see one that actual nails the waist mark.  Curious.