The most genuinely unnerving kind of moment in horror is that which takes you by surprise. It's why jump-scares are so often used, and why there are so many monsters you only see in quick flashes while they rip someone's jugular out.
And it's exactly this logic that has games that don't belong to the genre of horror still contain a terrifying area or two. As you aren't expecting to be spooked - and have no idea how long said spooks will last - any frights hit twice as hard as they would otherwise.
This can be oddly hilarious when a game decides to amp its horror for one single area, as afterwards you're left getting tense in otherwise totally normal scenarios, convinced that your nightmarish experience is restarting all over again.
Even years after you finish the game, you'll still have a moment every now and then where you remember the horrors that randomly appeared in an otherwise innocent title, and wonder whether you hallucinated the whole thing.
Except that's what the game wants you to think, dear reader. That's just what it wants you to think.