I understand it goes in the slot but arf's are not always cut exactly right that is why they have those measurements you are supposed to do and why you look at the level of everything. if the level is off you should sand it to level. Almost every arf i have seen has those check and sand steps just to ensure that it all goes together correctly. If the backside of your tail is higher than the front side then that could certainly cause your model to fly funny. I would certainly look there and look at the centering of your elevator and ailerons. if these surfaces are off they could cause a tail low attitude. I am not trying to insult but you posed a question and I tried to help.