The material specified is structural tubing, NOT pipe! It is actually 1.5" outside diameter, with the specified wall thickness.
One big clue is that they say .083" wall thickness, whereas pipe has a schedule (40 or 80 for example) instead.
(uh....yeah....see if you can find the drawing I copied this from!
http://usfirst.org/roboticsprograms/...nt.aspx?id=452
will help)