[mikst]

Apple due to it's market positioning has to put the most powerful cpu available. They weren't able to cool it though, so it was constantly throttled down, which is inefficient because you have to pay for the whole thing while you can only use 70% of it.

Now they put weaker chip but it can be utilized to its full capacity, so essentially at the cost of capital expenses, they significantly cut per laptop expenditure. Also internalizing more r&d helps with taxes.

I don't know whether potential instructions set optimization will yield that much, because Mac needs aren't really different; maybe they will focus on battery life more then other laptop manufacturers, which they historically did, who knows.