By now you should be at least familiar with Google Instant, a new search feature on Google. As users type in search terms, Instant predicts what the user is searching and instantly refreshes the SERPs.
Google Instant and Webmaster Tools
You’ve probably noticed an increase in page impressions lately. It makes sense that Google Instant is going to impact your page impressions. During the course of one search, SERPs update multiple times. If your pages appear for multiple “predictions” there will be an increase in impressions.
How Google Instant Measures Page Impressions
Before Instant Search was offered, page impressions occurred when the user typed in a word/phrase and clicked the Search button. With Google Instant, SERPs are updated as a person types. When someone pauses on a new results page for three seconds, that counts as an impression.
According to Google Engineer, Ben Gnomes, the average search query is 20 letters long and since Instant is basically search by character, not by word, this could result in a very noticeable increase in page impressions. It all depends on how fast someone types and if they pause for three seconds.
SEO Revolution Blog