90 degrees instead of angled terrain beside structures

When you add a structure inside terrain there is maybe a 10-20 degree angle of the terrain instead of a 90-degree angle. It leaves a gap between walls and terrain. It also means when I leave out a wall for a tunnel there is a gap between the terrain and the wall at all times. I'm wondering if the terrain could "touch" the structures? Or was there a reason I'm not considering keeping it at an angle?