When constructing the sets for the exterior hallways, someone forgot to tell someone else that Joey, Chandler, Monica and Rachel live well above the ground floor in their apartment building. Originally, the boys lived in number 4 and the girls lived in number 5. The producers finally twigged that these numbers were far too low for residents of upper floors, and had the numbers changed to 19 and 20. Of course, it still doesnt really work both apartments are the ones nearest the stairs, and although we never see the other apartments on that floor (because the set doesnt incorporate them), that would have to mean that apartment number 1 on the ground floor was the furthest away from the door to the building. It would also have to mean that there are only five apartments on every floor of the building, which clearly makes no sense whatsoever, unless the building itself is a very peculiar shape. Hmmm. Theres always the possibility that were massively overthinking this.