With Google Instant, page updates dynamically to show search results of what a user has typed, so this means people could be seeing and visiting your website much faster than before, and often without clicking search button or hitting "enter", as a result webmasters may notice some changes in Search queries data. "If a user types partial query and then clicks on any result on the page, that counts as an impression. That impression will appear in Webmaster Tools for the query, and the search term would also be included in the HTTP referrer when the user clicks through to visit your website," explains Google.
Impressions are measured in three ways with Google Instant: » Your site is displayed in search results as a response to a user's completed query (traditional model) » user begins to type a term on Google and clicks on a link on the page, such as a search result, ad, or a related search » user stops typing, and the results are displayed for a minimum of 3 seconds.
More Info: Search queries and impressions, CTR