Did you see the room when it was being lived in furnished? It might've been clutter all over the floor protecting the carpet from getting soiled.
But back to the soiled area. I'm seeing a combination of funk and crushed fiber distorting the carpet, making it look dirtier, even after you cleaned it.
And if you shine a blacklight on it, you'll see all over the floor where laundry detergent with brighteners was used sometime long ago to fk it up even more
