"Hull" or "Convex Hull" type of bounds is like having a duplicate of your shape that acts like a collision where every faces is
and where every face would be. Meaning if you had like a face on the end of your region missing there (like a doorway), you'll still get a collision there. Cyan's docs describe it as "Plastic Wrap" that covers the object.
"Exact" (which is the same as "Triangle Mesh" in Blender), is suppose to be just that: exact, so if you had faces missing, there would be no collision there.
Now as far as the "L" shape you got there, triggering in the area it's not suppose to, even with the bounds set to "Exact" or "Hull", I've run into before myself with Blender.
Normally what I ran into was the region's shape being very irregular or too many faces on it. This caused the bounds to act slightly strange.
However, You don't see too many "L" shaped regions in Uru, unless it's because there is a object in the inside of your "L" there. In which case the problem is moot.
Let's say it's a footstep region though. Let's say your green plane there is grass, but where your "L" shaped region is, is Stone. You'd have a problem there, but as suggested, you can fix it by having 2 regions, both boxed shape for the footstep sounds. It would work just fine.
Where you might have a problem is if the region is a trigger for something much more complex than footsteps or sounds. Using 2 regions then might not work out too well.
This is where careful planning of your Age before you build it comes in handy.
