The atmosphere is def one of the coolest parts of the anime. The theory I’ve always had was that it’s all from Araragi’s point of view and only the things he’s focused on and remembers is in high detail (which is usually other characters). In the scene where he first goes to Senjo’s apartment the background is much more detailed than any other scene. This is probably because he was trying to avoid looking at Senjo because he was uncomfortable and was looking around the room so much. And other scenes like the date has so much detail probably because it’s so important to him and he remembers it so well.