Do you know that, or is it your conjecture? There are a lot of other possible reasons to use $500 as a limit. For example, preventing people from structuring withdrawals at the ATM, preventing an accident from producing too much cash, limiting the frequency with which ATMs need to be serviced, and limiting the downside of unauthorized withdrawals.

It is true in some sense that the banking system is eventually consistent, because commands can be issued asynchronously. For example, you can issue a check, and expect it to be drawn in the future. However, that does not necessarily apply to the ledger itself, and given what I understand about banking technology, I'm somewhat surprised if it is truly as you describe.

If it is indeed a conjecture of yours, I urge you to make it more clear in your comments when you are speculating and when you are speaking with authority.