"Funds usually arrive in 3-5 business days, varying by bank"... why?

[sacka_man]

Whenever I disburse money from Amazon to my bank account, I get the following message in email:

Dear XXXXX@domain.com,

We initiated a transfer to your checking account (ending in ###) in the amount of $XXX.XX on 03/05/2013.

Funds usually arrive within 3 - 5 business days, varying by bank.

Why does it take 3-5 business days for funds to arrive in the bank? I feel like it should be a lot faster in today's age of instant information. I read on a forum it has something to do with ACH (automated clearing house) but I'm not sure and I was wondering if any of the experts here know anything about that. Thanks.

[rsot]

3-5 days is a guideline - some people have it in a day actually. The message comes up so that you can patiently wait...in case.

Not a big issue.

[GreenBean]

Agreed.

It's part of the game played by the banks.

Banks get interest on your money. No need to rush to release funds from their system.

[TonyAlmeida]

INGDirect transfer on Sat and Sun, both show in my account Mon morning