1script - 9:37 pm on Jan 29, 2013 (gmt 0)
I don't know if that explains the spread all that well - the bottom of the first page has always been a higher-CTR location than, say, #5-#9 on the first page. #1 on the second page would have also been a relatively high-CTR location for as long as they actually open the Page 2 (and, presumably, if they didn't open it, it would not count as an impression).
Deeper inspection shows that the site (which I believe was hit by both Panda & Penguin), is now ranking at the bottom of page 1 and on Page 2 for some low-volume keywords. That explains the increase of impressions while traffic remains about the same... at least for me.
Actually, come to think of it, this Page 2 thing may not be as straightforward - when someone searches for "blue widget" and only looks at Page 1 - do all those people that have sites in the result set, all the way down to page 100 (or whatever the cutoff is) - do they also see a +1 on the impressions at their relative positions?
It would have been ridiculous to do that for practical purposes on your own website but who knows how creative Google gets with their stats. For example, I have 200+ impressions for some KWs that show their average position as 880+(!) I seriously doubt that there may possibly be 200 people that got all the way up to Page 89 (or even 9 @ 100 per page) looking for that thing - this must have been either an error or an exercise in "creative statistics" by Google.
I have a nagging, not yet supported by any factual observations feeling that this 200% - 300% increase in the "Impressions" only means that there has been a fresh influx or pages from our sites that had entered the first 1000 results set (the main set as opposed to supplemental) on some (hopefully significant) number of new KWs. I don't know what to make of it yet. It does not sound like it should be a bad thing, and yet the total traffic is dropping.