I spotted a very interesting tidbit I did not know myself on how Google calculates organic search impressions within webmaster tools.

A Google Webmaster Help thread has a Googler explaining that a search result with sitelinks is considered one impression which has two individual listings from the same domain is considered two impressions.

For example, a search for this site returns a main result with a few sitelinks. But this is not considered several organic impressions, it is only considered one impression:

But when you have two separate results, then Google will count them as two unique impressions:

Asaph from Google said:

Did you know that?

Forum discussion at Google Webmaster Help.