The cat is skittish. A loud noise or quick movement will cause her to start and run off, sometimes with a pause to look at the perpetrator with utmost contempt. Maybe she was traumatized as a kitten.
But... there is one step on the stairs to the second floor where she doesn't. It's like that step is an alternate dimension... the Step of Invincibility. When she's on it, not even the loudest, most abrupt, most geez-I-nearly-stepped-on-the-cat-and-killed-it action will make her move, or even flinch.
Cats are weird.