Why are my Adword ads serving over my Budget?

They are not doing it if you look at it on a 30 days basis. They are doing it sometimes daily but it will adjust before you reach the 30 days. Maybe one day you have a budget on 2 $ and only spend 1 $. The next day Google Adword will try to adjust so that day you might spend 3 $ or something similar.

Google help section also have some answers you can check if you want to know more

How is my daily budget distributed?
Why am I being charged for more than my daily budget on some days?

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