My situation was earlier this year. The way I understand it is that the bank manager by some magic formula estimates the daily cash held about 1 week in advance based on past withdrawals history.
If you go in after lunchtime and someone has made a significant cash withdrawal in the morning the bank doesn't have enough cash to cover the afternoon. On further inquiry they also depend on daily cash deposits made by those frugal pensioners that hide it under the mattress to cover the shortfall in the vault.
If you are unlucky to hit a day of a major cash withdrawals and no-one has deposited any cash your out of luck and have to wait until a balance is reached or until the reserve bank sends some more, hence I put in my order for 'my money'.
