I'm sure you've experienced it too. You find a nice pair of jeans (on clearance, no less) at a retail clothes shop you can never usually afford, and they don't have any of your size left. What they do have, however, is a size smaller. Sure, you haven't worn a 34 inch pant since before you were old enough to drink (coincidence?), but what the heck, they're on sale, and maybe you'll lose some weight.
And, holy shnikes, they fit perfectly. Of course you buy them, and of course your jeans-shopping loyalty has shifted to the store where you wear the pants of a 19 year-old you.
Unfortunately, you probably haven't lost any otherwise unnoticed pant sizes. Instead, you're probably buying pants that are vanity-sized; that is, they are listed as a 34 inch waist, but are actually a half-inch to several inches larger.
Esquire magazine's Abram Sauer has an interesting post about the not exactly true-to-size world of men's pants waistline measurements with an accompanying graphic of his independent findings at one mall. As Sauer says, "You can eat all the slow-churn ice cream and brats you want, and still consider yourself slender in these."
