A Google Groups thread asks the question on why when a advertisers sets his AdWords campaigns to "Rotate: Show ads more evenly" in the campaign settings, does it not always rotate 100% evenly?
The advertiser asked:
The "% served" is skewed as follows:
Ad 1 75%, 25%
Ad 2 58% 42%
Ad 3 35% 65%
Why are these ads not displayed 50/50?
AdWordsPro Steph said that there are other factors involved when it comes to when ads are rotated evenly. Here are some reasons why an ad may not be served evenly with others:
For example, if an ad is created late in the day, it will have a lower ad served percentage at the end of the day than the existing ads. This difference will lessen as the ad gains new clicks and impressions.
Also, if an ad has yet to be reviewed and approved by AdWords Specialists, it'll only appear on Google.com. If it isn't yet generating impressions on the Google Network, an ad will have a lower ad served percentage.
Forum discussion at Google Groups.