Perhaps sandwiching the negatives serves the purpose of hiding imagesetter grain in the medium tone and (especially) highlight areas; imagesetter negatives can give serious grain; back when I was using imagesetter negatives, I had to print emulsion side up, in order to hide the strange blotching effect caused by some areas - being in very good contact - showing dots (even at 3600dpi hardware resolution!) and other areas not. This causes tonal differences and makes the print look blotchy / defective. Maybe they're laying the negatives so that the negative for the dark tones is in direct contact with the paper and on top of it the negative for midtones and to the top the negative for the highlights? (If they aren't doing multiple printings that is...) Since the 2nd and 3rd negatives (from the paper surface) aren't in close contact with the paper, they won't show the screening pattern, and since the detail / sharpness comes from high contrast areas which is (mostly) provided by the dark tone negative in close contact with the paper, this practice won't diminish (much) the apparent sharpness of the image. All that for creamy tones, is my take... (Again, if they are actually stacking all the negatives together and not doing multiple printings...)