Why are there more ads being billed than the preset monthly cap?
Caps may not necessarily align with the billing cycle. Caps are from month start to month end. So a cap may not be reached until after the billing cycle has closed, but still in the same month.
Example: A cap of 20 is in place, and 10 are posted before billing cutoff and 10 after billing cutoff but in the same month. Cap has been reached for the month, but the second batch of 10 will be billed in the following month. If all 20 of the cap are posted the following month before billing, there would be a total of 30 in the billing cycle, even though the caps were maintained.