My uninformed guess is that those are chips where one of the 8 cores failed in testing, but allowing them to be used in low-end machines means they won't be scrapped completely. Seems incredibly unlikely that they produced a distinct 7-core GPU variant just for that machine.

I wonder if the Air gets the chips with one faulty GPU core? They could just disable the faulty one and use the remaining seven. </idle-speculation> Edit: Whoops, I see I was not alone thinking that thought.