והא מבחוץ קתני קשיא – But the משנה teaches that it is placed from the other side! This is indeed a difficulty.
We can, with difficulty, answer the question of מבחוץ קתני. We can say that it means the חזית protrudes outside from the face of the wall. However the משנה then should have stated clearly 'מחוץ' – from the outside.