After taking apart said iMac, he concludes that Apple is using the extra aluminum from the knockout area where the display is to fabricate the new keyboard.
Bad Babelfish translation follows:
Apple seems that utilizes the opening component of the liquid crystal panel part which is cut to the occasion where it pressed processed the frame of iMac as the frame of Apple Keyboard.
Apple Keyboard (JIS) breadth, almost matching the size of the open part of iMac is because of that. Perhaps this, it is the kindliness for the earth of the apples style which exclude wastefulness with production process.
While I can't confirm that this is what Apple is doing, it sounds plausible to me.