Hmm I think all these replies are overcomplicating things.

At a group level, some kids are slower at this Stop/Go task than others. The group difference appears to be this increased broad-scale brain activity: the slow group is overall more prone to distraction and daydreaming.

However, at an individual level, slowing down on the task means increasing your focus (and decreasing brain activity in irrelevant regions), regardless of whether you were in the slow group or the fast group. So the group-level difference is not necessarily as profound as it might appear, and applying "slow group" with too broad a brush means you're going to sweep up some kids who are naturally cautious and focused.