Both options "Do Not Close" and "Edit" also contribute to the action count. The option "Not Sure" does not contribute to it. It's basically like a tracker for how many posts you took action on. You know, that silly word "actioned" in the Reviewer badge description. Since this doesn't actually keep track of how many posts you've looked at overall, I'm guessing that badge is going to change when the new system goes out of beta.
The same logic is applied to all the review tasks. Anything that's not a "Not Sure" or "Skip" increases your action count.