So I tried measuring my waist with a measuring tape, and I got 40". However, I wear size 36 pants. Size 38s have hard time staying on, and 40 just drop to the floor instantly. What gives?

So I tried measuring my waist with a measuring tape, and I got 40". However, I wear size 36 pants. Size 38s have hard time staying on, and 40 just drop to the floor instantly. What gives?