It's not even that, I did work experience with a bank, I know this stuff
So, if you have online banking, you are legally entitled to a "Balance" and "Available Balance"
Your available balance is how much you can actually spend, and how much you can withdraw. Now, when you pay for things on your debit card, your "Available Balance" will change straight away, but your "Balance" will stay the same for a few days. Once the card has been debited fully, takes about 3 days, your "Balance" will change to the correct amount, i.e, the "Available Balance"
This applys to Debit Cards, Credit Cards are much different, as you can have more "Available Balance", than your "Balance"
But don't worry, a tpyical cycle of paying via debit card/credit card is as follows..
Let's call it newegg, and halifax..
You place and pay for your order on newegg. Newegg then makes a connection to their finance servers, in which connect to the halifax servers, to ensure your details are correct, and that you have sufficent funding.
Halifax then notifies the newegg servers that your details are correct, and that you do have sufficent funding. The head halifax server, (super duper ones) then process your application, ensuring that the correct amount is taken out, and that the money is being transferred to the correct company, (newegg).
Now, there were, a few years back things called loopbacks. We're talking the 90's here..
A loopback is an error in which the company would recieve the money, and the money would not be taken out of your account, or the amount you thought you had payed for, actually gets refunded into your account, even though the original amount hasn't been paid. Best case scenario for you, you buy something for $1000, that payment goes through, and the item is delivered. Hey, next day, you are $1000 dollars up in your account. This normally happened when the servers were busy (i.e, alot of people spending)
There was one case in Manchester in which a cash machine was having faults, in which it was giving people an extra £60, and not debiting their account for it.
There were queues and queues for this machine, and overall, £60,000 was lost from the machine. It just happened to be one of the most stocked machines in the town. One gentlemen admitted he took his maximum withdrawful funds out (£250) in £20 lumps, in order to get as much money as he could.
Anywhoo.. don't worry, will be shown as being out of your account in the next day or two. But, the money has gone out!