Search queries has continued to grow, it is up 68% over the past 2 years. However, the number of paid clicks, clicks on ads contained within the search results,  has only grown 18% over the past two years. The graphs below show these two trends.

What accounts for sizeable gap between these two numbers? Why is growth in the volume of search queries outpacing the growth of paid ad clicks by a factor of more than 3x?
 
According to Gian Fulgoni of Comscore, the difference seems to reside in the fact that ad coverage has dropped from 64% to 54% of searches.
 
In other words, the percentage of search results pages that contain a paid ad has dropped from 64% to 54%. Why is that? Why has ad coverage declined?
 

 One hypothesis is that search engines have been improving the searcher experience and reducing the importance of less relevant advertisers. So, essentially, more relevant search results.
 
Backing up this theory, Gian looked into the rate at which searchers clicked on paid ads and found that the rate hasn’t changed at all over the past 2 years. 
 
But yet another theory according to Gian is as follows:

An analysis of comScore data shows that search queries are actually getting longer and that as searchers become more experienced they are using more words per search query. And this apparently reduces the likelihood that an advertiser has bid to have his/her ad included in the results page from these longer queries, due to paid search advertising strategies that limit ad coverage, such as Exact Match, Negative Match, and bid management software campaign optimization.