According to a discussion at Search Engine Watch Forums, Google's quality score plays an important role in ad delivery. A member has posted a response from Google which he got after seeing uneven delivery of ads when the Google’s Ad Optimizer was turned off.
According to the mail:
Regarding the serving, this actually is a common question that we receive. The system tries to equally rotate ads but there are other factors that weigh into this rotation. In your case it does look fairly even for the most part, however the system may be looking at the one ad as one with a less quality score than the other, thus not serving it as often. Although you have it opted into rotate more evenly it isn't guaranteed that the system will functionally work to an exact science. I know this isn't the answer you were hoping for, but this is what we have been told by engineering!
Earlier Quality Score was not considered in the event of Ad Optimizer being turned off. But after the recent Quality Score update by Google, Quality Score plays a role when Ad Optimizer is off.