(Conversation still to be visually digested. Maybe with model timelapse. LucasG 15:39, 9 January 2013 (PST))
Explaining the angles is nontrivial.
The H13 in plywood is not easy to build.
The H12 is self-correcting. The cone settles on the wall, it all trues up.
The H13 has to be built wall-and-roof as two bits and the front of the roof section, the vertical bit, wants to slide over the wall and fall to the ground when you lift one on to the other. It's perfect in foam, that works just fine, but wood's a whole other situation.