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Traffic is not constant from day to day. For example, fewer people search the Web on weekends than during the week. To account for this and maximize the potential of your advertising, Google may allow up to 20% more clicks in one day than your daily budget specifies.
If you budget $100 per day in a 30-day month, you may receive more than $100 in clicks on a given day, but the maximum you would pay is $3,000 for that month. If we do overdeliver clicks, you will receive a credit, which is listed on your invoice as an "Overdelivery credit".
You will get an Overdelivery Credit, but you will also get severe restrictions on the number of times your ad will be shown through the day.
These daily limits need to be watched regularly. You will often find that in a given day your ads get clicked when the expensive words in the pool are shown, so daily budgets can be easily surpassed.
I'm not sure on the exact cause of the discrepancy in your ads, but it might have been delays in the reporting.
As vibygor79 said, enjoy the free traffic.