Once Ads are live, the system will begin to score each Ad based on its performance and assign it a score. By default, the score for each Ad is scoped to the Zone in which the Ad is assigned to. This is an important distinction because an Ad can be assigned to multiple Zones but the performance in one Zone won't affect the scoring in another Zone. Ad Scoring is important because the system will choose the top scoring Ads to display.
The scoring system itself is directly correlated to an Ad's CPM but inversely correlated to the impression count of that Ad. As a result, you will find that an Ad with a higher historical CPM will have a higher score, whereas an Ad with more impressions will have a lower score.
To illustrate this you can imagine a circumstance where a Zone has two Ads: Ad X and Ad Y. Assuming that each has the same and constant CPM of $5.00, the Ad with fewer impressions will have the higher score unless the CPM changes for one Ad or the other. The same logic would apply to a new Ad in the system with a CPM estimate. Because of a lack of historical data or CPM, a new Ad can see a high rating at first and eventually drop lower once data is gathered and the score adjusts.
Overall, the correlation between CPM and impressions is the key to an Ad Score and will drive the Ad up and down in its likelihood of being displayed.