I had this happen to me in real life, back as a broke college student. The difference is the number of transactions you get hit with the overdraft fee for.

Consider a bank account with $20 in it, the day before payday, when you forget your World of Warcraft subscription hits today instead of tomorrow. That's $15. But you buy:

  * Lunch: $5
  * Coffee: $2
  * Dinner: $5
  * A late-night snack: $5

Then, the subscription hits at midnight.

If the transactions happened in-order, the subscription causes the overdraft, and you're charged $25 or whatever (yeah they are very high). But in the way banks used to do it, they'd take your total ledger for the day, and order it by highest $ first. So, subscription first, $5 left. Lunch, $0 left. Coffee, dinner, and snack are three transactions when overdrawn, so that's $25 * 3 = $75 in fees instead.

Banks claimed that this process was because you'd probably want your highest-amounts paid first, as it'd be something like rent money. In the end, it means way more fees for them, though.

$75 in fees is especially brutal when you don't have much money in the first place; it pretty much caused me to not be able to even go to 7-11 for a candy bar for two months.