Sets a cap on your monthly spending
We may charge you more or less than your daily budget in a day, but we will never charge you more than your monthly budget (daily budget x days in the month) for any given month.
It is not like they are trying to pull a fast one on you.
the Adwords program allows for daily charges that can be more or less than the daily budget. However, their FAQ says that they will never charge:
more than your monthly budget (daily budget x days in the month) for any given month.
For more information on daily and monthly budgets, see the Adwords FAQ, in the pricing and billing section.