aakk9999 - 3:09 am on Nov 20, 2010 (gmt 0)
in particular that the choice of additional pages seems to be rotating over time.
This is interesting - what it seems to me is that Google almost "allocates" how many additional entries will give to your site (on whatever criteria), so this is completely different than "indented entry" listing from before which just "joined up" entries from the same site that rank on the same SERPs page.
This case may quantify the above:
Our client is #3 on their main keyword. Positions #1 and #2 were taken by competitor's exact match domain, with competitor's #2 page being word for word copy of the text from our client's 3-year old (printed) brochure.
Three weeks ago the client has asked the competitor to remove copyrighted text and in all fairness the competitor reacted immediately and removed the page from their site so that the URL that was ranking #2 now gives 404.
Few days after removal we noticed that Google has replaced SERP position #2 with a different URL from competitor's site. Noticing that, we had a look at the competitor's site to see if there were any redirects or internal linking changes done from 404 link to the URL that now ranks as #2, but the removed URL was still interlinked on their site, giving 404 when clicked.
I have to admit that we did not expect is that #2 will straight away be replaced with another page from competitor's site.