Traffic is not constant from day to day. For example, fewer people search the Web on weekends than during the week. Google may allow up to 20% more clicks than your daily budget limit one day.
However, under no circumstances will you pay more in one month than your daily budget limit specifies. For example, if you budget $100 per day in a 30-day month, you may receive more than $100 in clicks one day, but the maximum you would pay is $3,000 for that month.