why did narrator said i was in a room i know but didnt knew

The narrator felt so because Mrs. Dorling's living room was furnished with things that belonged to her mother, her house. When she saw those familiar things she instantly recognised them, however, as they were now put up in another's room, she found everything strange and unfamiliar. Also, one can notice the distance she had already started to develop between herself and the things from her pre-war past. This sense of distance fully develops at the end, as is evident from her leaving behind her mother's belongings.