I don't think I have a satisfying answer, but to help understand how the underlying accounting works, take a look at https://en.wikipedia.org/wiki/Double-entry_bookkeeping_syste...

To expand a little, they take the cash and put it in the cash account. The cash account tracks the amount of real cash in possession. They then make a credit for that amount to your personal account, and a debit that amount from the cash account. I don't understand, when exactly, that corresponding debit happens.