I'd have to tinker with it to know for sure, but my guess would be Stop vs. Start. In a Stop story, the Problem is a massive, overbearing weight that needs to go away. Thus, an overabundance of Control. In a Start story, everything is reversed--the difficulty here occurs because the Solution isn't around. There's a Solution-shaped hole in the character's heart, and it needs to be filled. Thus, a lack of Control.
EDIT: There's also the discussion we've had every so often about how "lack of Control" and "Uncontrolled" are different things, but I don't think the app says anything in that regard. I'll expand, if you're interested.