The only reasoning I can think of is:

lead shot uses tighter chokes because the lead deforms a bit causing it to fly somewhat erratically and spreading faster. Steel shot doesn't deform, so it will fly alot tighter out of a more open choke.

If I remember correctly, Heavy shot (remington right?) Has odd shaped and oblong pellets of different sizes, supposedly making the pellets a bit more aerodynamic that spheres, but causes more spreading than steel. I wonder if it falls somewhere in between lead and steel shot for the amount of choke constriction needed?

I want to try some of this stuff this year for waterfowl, but I haven't had a chance to do any patterning yet.