How do you know the hands aren't flexible? How do you know they're made of stone and don't just look similar? If we're going to go with stone and normal, non-fantasy things, how does the blue girl even lift that hand, or move from that spot without dislocating her arms? I think you're approaching this pic in very much the wrong way.
As to the gym bag, it could be something she was carrying when she came to the world, and she hangs onto it and her uniform as the only tangible reminders she has of home.