Outer, Show and Hide boundaries have the option to be created as non-destructive breaklines. When this option is enabled it trims the TIN lines at the boundary. When it is not used it erases all the TIN lines that touch the boundary.
This can be a good option if you have good surface data on each side of the boundary as it will cut a clean and straight boundary through the surface. However, if this option is used on an outer boundary where all of the surface data is inside the boundary and the only triangles touching it are long and inaccurate, then you may be left with short triangles along the edge that are still at the wrong slope.
The graphics below show examples of the same boundary with the Non-destructive Breaklines option On and Off. As a general rule of thumb, if you want to draw quick Outer Boundaries that do not snap from point to point around the Surface, you should disable the Non-destructive Breakline option.