Here's the skinny. Your acct balance was based solely on pending transactions.
The pending transaction or current balance you had on your account showed authorized purchases that have not yet posted to your account.
To understand the difference between what's been authorized and what actually had posted, it's helpful to review the purchasing process.
If you use a credit or debit card to pay a merchant or you initiate a deposit, withdrawal or transfer to your account, you are authorizing the transaction amount. If it is a merchant transaction, in most cases, the merchant knows the exact amount of the transaction and asks you to authorize that amount. For example, if you buy $82.75 worth of groceries, the market would ask you to authorize an $82.75 charge or debit to your account. That amount would then appear in the pending transactions until it was posted.
Sometimes, though, the merchant doesn't know the transaction amount until after it's authorized. A good example of this is at the gas pump. Before you fill up your tank, you're asked to swipe your debit or credit card to authorize your purchase up to a specified amount. Say, for example, the authorization request is for $50. This amount also shows in your Account Summary as a pending transaction. Your gasoline purchase ends up being $22.50. That's the actual charge that will be paid to the merchant. In rare instances, the actual transaction is posted to the account before we receive an updated list of pending transactions. In that case, it may look like you're being charged for the authorized amount of $50 and the actual purchase amount of $22.50. In reality, the pending charge will usually be removed with the next update.
It works the same way for all transactions. Transactions are not ran through in the order they were received for payment. Pending transactions are all ran through at the update time - usually midnight for most banks on a business day. It may not be in the exact order they were submitted for processing. Most banks go ahead and apply whatever it is to your account, but if there are numerous things that are going to hit your account, the bank will take the largest amount first. That's just the way it is. The bank is there to make money and the largest money maker for banks is not interest (even though they'd like the american consumer to think so) It's FEES. Believe me, I work for one of the top 3 banks in the world. FEES FEES FEES