When you create an account budget, the amount you're billed, across all your campaigns, won't exceed the budget amount you choose. But the amount spent (called the spend) can be higher.
Discrepancies between your actual and requested account budget can happen if a credit is applied to your account budget after you create it. That's because your credit becomes part of your actual account budget. And once that's used up, all your campaigns will stop and no ads will serve.
To prevent your ads from stopping, you can create another account budget. Or you can add more budget to an existing account budgets by clicking the "Edit" link next to the order -- if the order has not reached its end date, that is.
Let's say you create an account budget for $100,000. You won't be billed for more than $100,000. Now, let's say a $1,000 credit is applied to your account budget. This means that the actual budget becomes $101,000. This would explain the discrepancy between the actual account budget ($101,000) and the requested account budget ($100,000) that you'll be billed for.
Monitor your budget's amount and spend during the period of the account budget. This can help make sure your campaigns don't unexpectedly stop running.