wow, no shit facebook??
Hi Josh,
For any given ad unit, we select the best ad to run based on the cost per click or cost per thousand impressions and ad performance. We have a process in place that will automatically calculate the minimum price that the advertiser could pay and still have the highest cost per click or CPM ad, and the advertiser will only be billed that price. This price may be below the advertiser's maximum cost per click or CPM. Because we lower the cost per click or CPM on your behalf, we recommend that you enter your true maximum cost per click or CPM when creating an ad. This will increase the likelihood that you do not miss out on clicks or impressions that you otherwise could have received.
Let us know if you have any further questions.
Thanks for contacting Facebook,
Rose
Customer Operations Representative
Facebook