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Say my limit is $10 per day, and there are 31 days in the month, then conceivably I could spend $310 that month.
I only run my ads on Friday - Sunday because that is when the physical store is open. So if the first Friday falls on the 6th of the month, I have found that Adwords will play "catch up", and will spend up to $60 that first day because it knows it's under my monthly budget, and then will slow down to normal. When I stop the ads on Sunday it will play catch up on the following Friday again, and so on through the month. It doesn't pay much attention to my daily budget because it seems to go by what my "month to date" limit is.
As a result, and the end of the month it takes the number of days I actually ran the ads and multiplies it by my daily budget to find the maximum it is allowed to charge me. Anything over that seems to be refunded as an "overdelivery credit". That's nice, but it is also a pain because I never know how much I am going to actually pay until they bill me.
This may be what is happening to you as well if you started running your ads in the middle of a month or don't run them all the time.
If the AdWords system is not able to 'balance' the day-to-day spending, then anything over your budget is credited to you just before you are billed. If this happens, it amounts to free advertising for you, as the traffic has already been delivered.
Please see #6 on this page from the AdWords FAQ for a bit more detail: