Of course the 1060 is based on GP 106. However, as I said before in order to make those they would literally have to stop making 104s because of the poor yield situation (which is obvious looking at stock levels).
So, why would Nvidia stop making a card that costs hardly any more than one they will sell at half the price?
As such some of the cards that have gone out to reviewers are cut off 104 dies, not 106.
I've also said that this would be a vapor launch because of the above facts about supply issues and so on.
However, it kind of shows just how greedy they are being with the 1080 and 1070 if they can, like you say, afford to just lop a lump off to make review cards.