Yes, boys are different from girls. I'm sure that your girl is rough and tumble and can keep up with the boys, but the difference is boys never stop moving. Even when they are still they are moving. Boys can't walk by a wall without running their hands across it, they can't watch cartoons without wiggling some body part, they don't talk when they can yell, they don't walk when they can run....you get the idea. These are my son's knees. Definitely not the worse knees he's had this summer, but up there.
I scrubbed these knees, they looked the same, I scrubbed them again, and they still looked the same, so I was desperate. I experimented and the only thing that got the dirt out of his knees was MAC eye makeup remover. Now I have two uses for one of my favorite products.