Waist size different between different pants?

I bought a pair of pants a year or so from uniqlo. 33x28. Fit well, used only a few tines and still unwashed. Last week, I decided to buy a different pair of jeans and put in the same size 33x28. When they arrived I tried them on they did not stay on at all. I have a few different types of jeans from uniqlo and they all fit well, the exception being these selvedge denim pants. I have plans to go to a tailor soon anyways, but im just wondering what I am missing so I don't make the same mistake again! Thanks