Google has confirmed the reports of a bug where configuring your search preferences to non-instant results, set to show 100 search results on a page, stopped working.
The bug began sometime around October 18th, when we first noticed a Google Web Search Help thread with complaints.
Google's Nealeigh only responded to the issue a couple days ago saying:
Thanks for your reports! We're looking into the issue. I will provide info as soon as I'm updated on the situation.
Google may have just rolled out a fix, as one user is now reporting the preference sticks and he is able to see 100 search results on a page.
Forum discussion at Google Web Search Help.