De-schooling

I understand from parents who unschool that it's pretty much a universal principle (if that can even be said about unschoolers)

(it can't)

that children who are pulled out of school and set to "unschooling" (given freedom to do more or less whatever they want all day) all go through a period of adjustment in which they don't do much at all. They lie on the couch and play video games, or whatever the activity of lowest resistance is.

Eventually their natural curiosity kicks in and they begin to learn... whatever they're interested in, and that leads them to other subjects, and they grow up, the unschooling theory is, eventually well-rounded and passionate intellectual seekers.

What's interesting to me is that this "de-schooling" period is so universally predictable. There's a formula for it: a child removed from the school system and set to unschooling will go through one month of "de-schooling" per year that s/he has been enrolled in school. Unschooling parents can set their watch by it.

What's behind that?