I spent a big part of my working career working for a freight auditing company (I wrote programming to allocate funds to departments within companies).
I learned a lot about money there. One thing that some freight auditing companies do (ours didn't on purpose) is to "float" the client's money. They do this by gathering all the bills for a client from all the freight companies they use, ask the client for one big check, then they pay the freight companies. It seems like a nice service, except they take a few days to pay each carrier and when you are dealing with hundreds of thousands to even millions at a time you are gaining money each day it's sitting in your account.
Now, not to be a conspiracy theorist but I wonder if the bank that you are transfering money from just takes their time for some reason like that. It seems to me that as soon as the recipient bank gets the info the money is in your account immediately.
Another thought is how when you go to a department store and buy something on your credit or debit card the money is almost instantaniously out of your account. However if you need to return merchandise and they put it back on your card it can take 2 weeks! I think that is the banks fault not the department stores.
My answer is: I don't know, actually, I just couldn't help but share some thoughts that may or may not have anything to do with the question.