As you can see in the below picture, I set 25 result to be shown per page but when I click on searching for the new posts on my forum I still getting the default value of search per page which is 20.
Since this is a Search Result Page not a Discussions Per Page (This controls the maximum number of discussions (such as threads) that will be shown on one page.)
Well Chris, it still a search page whether are the content a threads or posts, I should be able to control the number of search per page from that option page (Search option) because it doesn't make sense when I change the Maximum Number of Search Results (From 200 to 500) from there and the number per page I have to change it from an other page option!!
It's a discussion list, so we use the discussion list per page option. To make it use a different value would require a code edit or an add-on, or custom development.