I'm slightly confused, how can I have only around 5 Adsense impressions (according to Adsense statistics) per 15 page views that came from Google search (according to Analytics statistics).
One thing to consider is that Adsense is PST, Analytics is keyed to YOUR time zone so the periods of reporting are different.
There is a large difference between adsense and google Analytics and AWstats on my site. Often adsense will show between 10000 and 20000 less page views per day than what awstats shows.
Awstats, if I recall, counts any access to the page, thus is will normally be much higher than adsense which counts only real visitors (or at least those with javascript)