It is all about the actual triangles used by the render engine. Go to the mesh component display and enable "Display Triangles" and you'll see only the one triangle is deformed. If you move the bottom right UV then both triangles are deformed. The deformation will be "smoother" and more even if you increase the number of triangles. When you hit 3 to "smooth" the mesh you are adding additional subdivisions hence increasing the number of triangles.

With triangle display enabled hit 3 and then play around with the subdivision levels under "Smooth Mesh" as well as the Smooth Mesh->Extra Controls->Map Borders attributes.