I could be a mixture of some or all of them, but it seems very suspicious that 1 alone can explain it.
I am in complete agreement and welcome the insight that may come from comparative testing in due course. Also, I would add one more factor to your list of possible explanations. Because ARM has not faced external competition for a long time, their ecosystem may have grown complacent. The rigors of competition may force all to up their game. That is good news for consumers notwithstanding who prevails.