Remember, you're reading rumors, there were a lot of "likely" and "expects" in there, not facts.
Apple has THREE different 13" MacBook models right now (Air, Pro and Pro with Retina), so additional fragmentation really isn't so weird. Keep in mind, they used to have 12" PowerBooks and 12" iBooks and I believe they had a 14" iBooks while they had 15" PowerBooks. I think it is more likely Apple could be transitioning sizes.
I could see Apple doing a 12" Air and ditching the 11" and 13"? Maybe making making a 12" retina Air and keeping one or two of the others?? Or dump the 11" + 13" and just have a 12" Air and a 12" Air w/ Retina?
It just depends where they can get the pricing, they always like keeping a $999 model.
I don't see how they'd do a 12" Air Retina and make it significantly thinner than current models; although they iPad Air is 50% heavier than an iPad 4, that was a lot to shave off!
The options are staggering when it's just rumors...
Gary