I’ve noticed this strange behaviour for a while but have only now had to time to try and find out what is going on. When I’m signed in to a Google account that has Google+ associated with it I cannot display more than 10 search results per page. This is despite having Instant switched off and specified 100 results per page in my search settings. I checked another Google account that does not have Google+ and my settings are respected as they are when I am signed out. I have cleared cookies and web cache, and tried different browsers. The same problem occurs. So it seems that if your account has Google+ associated with it Google overrides what it likes! I already have two browsers open all the time: one signed in to my main Google account for gmail and other stuff, and one signed out for search. Sometimes, though, I want to run the same search within Google+ and then on web search so I have to go to the effort of copying my search across to the signed out browser.
This may sound like a minor issue but if Google is ignoring this user setting one starts to wonder what else it is choosing to ignore.