Two people can spend identical amounts on identical days and be charged very different overdraft fees. The difference is not the spending but the order in which the bank posts the day's transactions.
Authorization and settlement are separate events
When a card is swiped, the merchant asks the bank to authorize the amount, and the bank places a hold. No money has actually moved at that point.
Settlement happens later, often overnight and sometimes days later for merchants who batch their submissions. The posted balance changes only when settlement arrives, not when the purchase happened.
That gap is why an account can display an available balance that already reflects a purchase, while the ledger balance the bank uses for fee decisions still does not.
Banks process a day's items in batches
Rather than posting each item the instant it arrives, most institutions collect the day's credits and debits and process them together in a nightly cycle. Within that cycle, an order must be chosen.
Some banks post credits first, then debits from smallest to largest. Others post the largest debit first, a practice known as high-to-low ordering.
The choice is a policy written into the deposit agreement rather than a technical necessity, and different institutions have settled on different rules.
High-to-low ordering multiplies the fee count
Suppose an account holds enough for four small purchases but not for one large one. Posting the small items first clears them, and only the large item overdraws, producing one fee.
Posting the large item first drains the balance immediately, so all four small purchases now overdraw. The same spending produces four fees instead of one.
Nothing about the customer's behavior changed. The multiplication comes entirely from sequencing, which is why the practice has drawn sustained regulatory and litigation attention.
Deposits do not always land where you expect
A deposit made during the day may post in the same nightly cycle as the debits, or it may be held under the bank's funds availability schedule and post the following business day.
Check deposits in particular carry holds tied to the amount and the age of the account. A deposit visible on the app is not necessarily a deposit available to cover a debit.
This creates the common experience of covering an account before midnight and still receiving a fee notice the next morning.
Opt-in status decides whether the item clears at all
For everyday debit card purchases and ATM withdrawals, a bank may only pay an overdraft and charge for it if the customer has affirmatively opted in to that service.
Without opt-in, those transactions are simply declined at the register with no fee. Checks and recurring automatic payments sit outside that rule and follow the account agreement instead.
The practical consequence is that two accounts at the same bank behave differently on the same transaction, depending on a single election made when the account was opened.