In addition to showing him the pictures, you can explain to him that the difference is in the manufacturing process. A milled receiver starts as a block and has material milled away until you are left with the final piece. A stamped receiver starts out as flat pieces, and is shaped by being pressed into its final shape.
Generally milled pieces will have closer tolerances and be stiffer, but in Ak's loose tolerances seem to be part of the design, for reliability. Stamping is also cheaper and easier.