Discrepancies between your actual and requested budget orders
When you create a budget order, the amount you're billed for that order 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 budget order can happen if a credit is applied to your budget order after you create it. That's because your credit becomes part of your actual budget order. 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 budget order. Or you can add more budget to an existing budget order 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 a budget order 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 budget order. This means that the actual budget order becomes $101,000. This would explain the discrepancy between the actual budget order ($101,000) and the requested budget order ($100,000) that you'll be billed for.
Monitor your budget order's amount and spend during the course of the budget order. This can help make sure your campaigns don't unexpectedly stop running.