I have a theory, that Ann and Anne just roll right off the spoken end of any first name so neatly, you can glue it to any name and have it work, that's why it's The Universal Middle Name. It's just easier, saves new parents lots of work. It's hard enough coming up with a good first name. Nowadays they won't even let you out of the hospital till you've legally named your baby. So sometimes you have to hurry, if you haven't thought of anything.
My ex-husband was never given a middle name, he really appreciates the general simplicity of that. I think his parents just forgot but I've never told him that.
The deep-manure-double-name-syndrome; it really packs a punch, that's why they use it. It's the automatic signal that you are Total Toast, Go Straight To The Dungeon Do Not Pass Go. It's so automatic, it's a real time-saver, no discussions needed. Because you yourself actually know why two names are being employed.