The day after the first wash, when I had the jeans laid out on the table to photograph, I noticed these "wear" marks just below the back pocket on both sides. The top photograph has two pictures. The left one from before the wash and the right hand one from after the wash. You can see them on the jeans before the wash, but they appear to just be folds, or creases. After the wash you can clearly see the denim is damaged, meaning really worn in. It must be from each time I sit down, when the denim folds and then flattens out again.
It's probably cause I'm so skinny. If I had a bit more meat on my bones, or specifically around my bum, the jeans would've been tighter, hence less creasing. Not that it's problematic or anything. Just a point of observation. And I'm really glad my mom doesn't read this blog or I'll soon get an email, telling me to eat some more food, cause it's not healthy being so skinny. hehe.