My site doesnt show results for "film reviews" when searched for on the page but shows for "movie reviews". I searched for "film reviews" when inside the search result page and then results came up.

Does this mean that when people search from my search engine on my site that they actually search within my site? It cant be as when i search for "movie reviews" regular results come just as by visiting google.

Can someone tell me why this is happening?

My site doesnt show results for "film reviews" when searched for on the page but shows for "movie reviews". I searched for "film reviews" when inside the search result page and then results came up.

It just means that (for Google) the page is more relevant for "movie reviews" than "film reviews".

when people search from my search engine on my site that they actually search within my site?

That depends on which radio button is selected under the search box.