AdWords Can Bill You More Per Day Than Your Average Daily Budget Setting

For the month, you won't pay more than your daily budget times the average number of days in a month. Some days you might spend less than your daily budget, and on others you might spend up to twice as much.

When you first start up your new campaign, set daily budget lower until you see results. Otherwise, you can spend too much before you know competitive bids and how many people put your words into Google search. Tune your Ad Group bids down until your actual daily spend is less than your daily budget. Then you know you know that Google is running your ads all day, since you set Delivery to "Accelerated"  Google will average your spend throughout your month, so your ads run all month.