This effect can be large enough to push your overall average position down.

We have used a statistical model to account for these effects and found that, on average, there is very little variation in conversion rates by position for the same ad, continues Varian. For example, for pages where 11 ads are shown the conversion rate varies by less than 5 across positions. In other words, an ad that had a 1.0 conversion rate in the best position, would have about a 0.