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?

Gravity.

Focus, that's hilarious. LOL.

and is your stomach in the waistband of your pants or that oh so attractive look that entirely too many men have where it's hanging over the pant waistband (it's kind of like a combover

Maybe men's pants have now gone the way of women's clothing -and are all about vanity sizing... where clothes are just being made bigger so that people will buy more...

I find there's a lot of difference from company to company. I go from 36 to 40 depending on the brand.

I don't measure. I just go by how my clothes fit. It's the best feeling in the world to go down a pant or panty size! lol

Don't worry about inches much..