tedster - 6:37 pm on Dec 3, 2011 (gmt 0)
What's even weirder is I get a different set of results for 20 per page to 10 per page (ie the top ten results are different).
This has long been the case - because of the way URLs from the same domain can be clustered. If you ask for more than ten results, there can be more URLs from the same domain to be clustered, and they usually cluster around the highest ranking result. This means a result that was at #14 can pop up to #3, for instance, it the same domain has a result at #2.