I've always sat in weird positions. In school, I'd sit on my chair with my legs crossed. I do that to this day, and I'm sitting like that now. Sitting cross-legged on chairs is more comfortable than sitting with my feet on the floor. I honestly don't understand how people can sit on a chair "normally" and tell me that that is comfortable. It's not. It feels unnatural.
If I'm not sitting cross legged, then I'll either have my feet resting on something, or I'm sitting with my knees up to my chest, and tilted to the side, usually the left.
As for leaning, I never noticed until recently, but when I'm in a car, I rarely sit straight ahead. I usually lean to the left, or my body is slighly turned that way.