Here is the main reason. DoubleClick is reporting impressions served or delivered to ad tags. Analytics packages like Google Analytics of Adobe SiteCatalyst are counting the execution of page tracking code.
If your Google Analytics tracking code is at the bottom of your web page and placed right above the closing of the body section (see Figure 1 below) then you are definitely at risk for reporting discrepancies. And, the bigger and heavier the page the more at risk you are.
|Figure 1: Google Analytics code placed at bottom of web page HTML code.|
And if you think about it, people will bounce more and more quickly on ads versus search results. Right?
So, what is the solution?
One simple solution is to place your Google Analytics tracking code near the top of the page to minimize the risk. Simple fix.
And, in fact, when you create a Google Analytics account today, Google tells you to place the code at the top of the page right before closing our the header section. That is not what they used to tell us. Hence that is why when viewing the source code for most websites you will see the tracking code at the bottom.
So, if you want to minimize the risk of your ad server impressions not matching your Google Analytics or Adobe SiteCatalyst figures, just move the code to the top of the page. It will not remove the risk completely but will come close.