The others have hit the nail on the head. I often call it a "puppy sit" as you will most often see it in puppies before their coordination and precision is polished. As a youngster, I wouldn't worry about it. But I do like to encourage a proper sit once the dog nears 10 months old or so, so that you don't get into any bad habits later down the line.
That's what I call it! It seems to go away around 6 months or so, with my shelties. (Hence the name "puppy sit!)
After they'd developed stronger muscles in their back and hind legs it was no longer a problem. They are both sitting properly now, so don't worry! 