One sentence is 'I took the job on the isolated island'. The other is 'I didn't know the islanders were cannibals until they started sharpening their knives for me'.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
I think 'The clock stopped at midnight. Then I heard footsteps.' is quite scary. It gives this feeling of something ominous starting right at a spooky time.
Some two - sentence horror stories are truly spine - chilling. For example, 'I woke up to a noise in the kitchen. When I got there, all the knives were floating.' The simplicity of the setting and the unexpected, supernatural element make it effective. Another one could be 'She heard a baby crying in the attic. She doesn't have a baby.' It plays on our primal fears of the unknown and the unexpected in a very short space, creating a sense of unease and horror.
In a way, they can feel real because they touch on primal fears that we all have. But no, they're not based on actual happenings. They're just creative expressions to give you a quick scare.