Translation:You are working at the office, I am working in the kitchen.
This is hard to explain. As a native speaker, it's just one of the things we do. If you said working 'in the office' that would mean like a home office. Similarly, you would say 'I'm AT home' but 'I'm IN the bedroom.' So AT is like for a place, but IN is for a place within a place. Ex: "I'm at the store, in the bread aisle." In the end, IN is usually ok and doesn't make much if a difference.