While alpha testing the new Search Impact feature in Google Webmaster Tools many of us noticed the impression data discrepancy. Dejan SEO website (web search only) shows 1,539,133 impressions in the Search Queries report while Search Impact shows 1,506,534 impressions for the same date range.
John Mueller explained the cause for the data discrepancy in a comment to a post by Gyi Tsakalakis.
“…impressions looks at the site, not at the URLs. So if you have a query where we show 3 URLs from your site, the old one counted that as 3 impressions, while the new one shows just 1 impression.”
This means that if you have multiple pages from the same domain appearing in Google’s search results they will count as a single impression. For example, on this page I have a triple listing, but they count as one in the Search Impact report.
So there is no need for alarm once this feature goes public. In fact I found it kind of useful. By comparing the two figures (1,539,133 – 1,506,534 = 32,599) I found that multiple page listings for Dejan SEO appear twice for every hundred SERP impressions.
That’s a neat little piece of data I didn’t have before!