Galut - What is Considered a Woodchopper Case?

Last night we began a new mishna on Makkot 8a.

Before starting the mishna, we gave a brief introduction to the different categories of action and what the Torah's response/consequences are for each action:

אונס - totally uncontrollable situation - unforeseen circumstances, no גלות, no death penalty
שוגג - "accident" - of the type in the Chumash in the Woodchopper situation - גלות
קרוב למזיד - negligence, more severe, higher standard of care than woodchopper case - no גלות, no death penalty
מזיד - deliberate murder - with התראה - death penalty

The mishna says that one who throws a rock into the public domain goes to an עיר מקלט ie galut.  This is troubling to the Gemara - which instantly exclaims - מזיד הוא! We explained that the Gemara doesn't literally mean that the person has committed a level 4 offense for which the action would be punishable by death.  Rather, the Gemara is saying this is an action which is קרוב למזיד which גלות does not redress.  Why, then, does the mishna say that he goes to galut?

The Gemara's answer is that the case of our mishna is that he was destroying his wall (and a rock flew off into the Reshut Harabim and killed someone) Apparently, this is meant to reduce the קרוב למזיד aspect, and to align it with the woodchopper case of the Torah. The question is how? One explanation is that, like the woodchopper case,it is not inevitably going to cause a death; it is a productive activity that entails risk, as does the case of the Torah.  Therefore, when the accident does happen, it is considered שוגג and the person is sent to Galut. 

We finished the class with the Gemara's objection to this answer, by noting איבעי ליה לעיוני - he should have paid attention/looked out! The Gemara feels that the "wall" case therefore is dissimilar to the case of the Torah, and the question is what is the distinction to which the Gemara is pointing?