leapforward - 3:03 am on Mar 22, 2006 (gmt 0)

I've seen this happen to a few of my sites. Here is my theory on the CTR bump for first serp.

1. more likely that the user has just begun their search for the term
2. they believe that the very, very smart search engine said that your site is the best for their search.
3. your site is the first one the user clicked on the SERP.

Given all of the above, a user is more likely to give your site more of their time and thus click more of your ads.

if your site were the 5th clicked in the SERP, the user is now a bit more educated and frustrated because they haven't found what they are looking for. The user is likely to spend less time browsing your site looking for what they want. this leads to lower CTR.