@gjm:

Just wanted to say that this is well thought out and well written - it is what I would have tried to say (albeit perhaps less eloquently) if it hadn't been said already. I wish I had more than one up-vote to give.

@Eitan_Zohar:

I would urge you to give the ideas here more thought. Part of the point here is that from you are going to be strongly biased for thinking your explanations are of the first sort and not the second. By virtue of being human, you are almost certainly biased in certain predictable ways, this being one of them. Do you disagree?

Let me ask you this: what would it take to make you change your mind; i.e. that the explanation for this pattern is one of the latter three reasons and not the former three reasons?