Update:
I did some more beer can testing this morning with the seax to verify Jeb's theory. It appears that the slant does indeed force the edge in the opposite direction during a thrust; it seems that this action turns the thrust into kind of an internal draw cut, if that makes any sense. Whatever the case, the holes left are considerably larger than the blade but only the initial point of impact is triangular, with the bottom half of the hole being more of a slit.
Thrusting edge-up seemed awkward to me and I couldn't manage as much penetration but this is probably just a matter of practice. I observed the same thing - edge forced upwards, enlarged hole.
All of this was duly recorded but it's pretty gloomy out today and I won't know if the videos are any good or not until I get them on the computer. I'll try to put some up later today or tomorrow.