You can check out information regarding Daily Budgets from the Google Help Center. http://adwords.google.com/support/bin/answer.py?hl=en&answer=6312
when you set a daily limit you are actually setting a monthly limit. you are telling google i am willing to spend $10/day for 30 days. What they do is smooth that spend across 30 days so it averages $10/day. In the short term google might sopend more then the $10 but over the month it will average out.
That interesting, i wonder how they determine to go past the $10 daily limit. Maybe when there's heaps theres a huge burst of clicks for your add or something
google allows for a 20% daily flux so if your daily is $10 then they will flux daily between $8 and $12 and aim to spend a monthly total of $300. If they go over their 20% they will issue a credit