I saw a post years ago referencing the variations even among the exact same size and pattern jeans. The jist of it was, due to the way the fabric is stacked, as the die presses down through it to cut the pattern into the stack the fabric stretches/gives different amounts depending on where it is in the stack, it can be really exacerbated by dull blades too.

I don't even know how true it is but I've also noticed the wild sizing variation in jeans, even relatively good brands. You really have to go try them on if you want to make sure they fit the way you want.