Literally the second sentence of the article. On top of that, units in the same building often cost vastly different amounts of money depending on the height, the view, etc.

I've seen buildings with separate doors and some without that have low income units. Even in the same building, the low income units are not as nice (e.g. no in-unit washer dryer, no stainless steel appliances, cheaper fixtures). Also, some buildings only provide keys to the gym and other common areas for non-low income. But I think they got in trouble for that and may have stopped.

My guess is that the rents in the building are not uniform.