[Bf-funboard] Differencing Cubes

[Roger]

<numpad1>
Create a Blender cube 3 blender units square
scale cube on y axis to 0.5 blender unit.
Deselect

Create a smaller cube in the centre of the first cube to extend along the y axis
past the 0.5 thicknes.
Select cube 1 then cube 2
'W' Difference the cubes
  Move the original cubeset away

What is left is a hole in the first cube
But
For some reason verts  have been added  to edges around the removed space 
creating unnecessary edges added to the object connecting these extraneous verts 
to corner verts of the first cube.
It seems that blender has to make it into triangles when quads are required.
Why ??
There is no need to add verts along an edge, cut edges or make triangles.

Is this a bug or am I doing something wrong.
TIA
Roger