Why does WordPress search returns same number of results for every search query?

I’m pretty sure that it’s pagination problem.

WordPress paginates all queries by default. And default value for posts_per_page is 10, I guess.

So if you don’t override this parameter and you don’t show pagination on your page, then you’ll see only 10 results (first page of results).

Is it your own query? If so, add 'posts_per_page' => -1 to it.

If not, add pagination to search results page (or change number of posts per page in settings).