I know a little something about this. I've measured tons of pants, both inseam and waist. Tag sizes are really just a suggestion. Factories cut the patterns with room to sew everything together. They don't measure pre-sew, so there's some wiggle room (+/- an inch, usually). Patterns are cut by machine, but still.
For jeans, consider the tag a guide. Inseams are often 1-2" shorter than stated. Waist is usually pretty close. Especially if you account for pre-shrunk versus raw denim.
Spandex ruins everything. They fit great when you first put them on, but stretch out as you wear them. Ugh.
Tell me what brand of jeans you're talking about, and I can give you my insights.
Some brands do "vanity sizing"...to make you feel good. Like, you're a Large now when you used to be an XL. Tommy Bahama does this, at least with their shirts....