Very interesting. I don't think I ever would have thought about that.
My particular smoker has a fire box underneath "the belly". It has a steel plate also that runs along most of the bottom underneath the grill grate (a little like a reverse flow but not really...). I am thinking that after the smoker heats up pretty good, that middle section is considerably hotter than the rest.
Home-made 6 1/2 ft trailer smoker; WSM 22.5