This is because the developer did not tag the platforms for their game. The app can't tell what a file is for unless the developer selects the operating system for the file after uploading it. The platform selection is not related to whether it shows up on search. But if a developer fails to tag their files for a platform, then they won't appear in that particular platform's browse pages.
The game is not appearing in search for anonymous users because we do not show NSFW/18+ content by default. An account is needed, and that account must opt in.