I too thought it was going somewhere else at first. I thought we were getting a character piece like Patrick Suskind's "The Pigeon" (about a security guard living a very modest, very regimented life, whose routine is upset by a pigeon, driving him near insanity). It diverged when the neighbors got involved and it ceased to just be about the protagonist.
Like most good horror stories that don't seem to make sense, the crazy shit makes sense if you think about it metaphorically. It doesn't have to be literally coherent to be metaphorically coherent. (See also: every religion ever) The apartment that isn't hers with the other tenant, and the operator's reference to never having been down to that point far above where she had thought the building ended, represent her feeling that there's something better that she'll never be able to reach. She had to stretch herself to the limit to get 37 floors away from the things that she feared, and she knew there was farther to go. It was a comprehensible distance, both physically and socioeconomically. She's fallen into a realization both that even that isn't as far as one can go, and that perhaps she's gotten closer to something else she from which she might want to distance herself.
If my interpretation of the metaphor is somewhere close to correct, then there's some similarity between the endings of this story and "The Pigeon," in that both protagonists end up accepting their new relationships with reality, though for one it is (IIRC) with relief, and the other with resignation.
I get the idea that this story is more about the fears of someone who lives in a building like that, rather than the fears that drive a person to live like that. It's a subtle distinction, I think, and I'm more interested in the latter; but I can respect it for what it is, I guess.