For me, I think @wildflower nails it. Gap, Bill’s Khaki’s and a slew of other places will sell you a pair of khaki slacks (or jeans, or whatever) modeled after the old WWII-era style… with the waistband running right up to your navel.
I’m not James Bond (wearing classically cut suits/tuxedos), and so I prefer a lower rise (the distance between the crotch and the waist of a pair of trousers) in my pants and like ‘em to hang low on the hips. The hips, as wildflower points out, are actually wider than the waist.
So the pants you buy might not be measured for the waist, but actually for the hips.
The point is, it could be a correction made by the manufacturer based on where they intend for the pants to rest. But as the entire situation is an arbitrary standard applied ad-hoc per manufacturers whims, it’s easy to perceive that things have slipped.
I’m just glad I’m not female. What the heck is a “size 4”, in terms of height, weight, width, etc. and how can that ever describe any particular body dimensions exactly?