Wall alignment

I must be having a brain block! Thats the only thing i can think of atm to explain why i cant get these logs to line up!












The corner pieces line up fine when mirrored which is how i was going to place them in the scene. i took the open edges and duplicated them across to create the wall and for some reason it lines up fine on one side but is slightly out on the other side! i cant explain why. its exactly the same on both sides, maybe its the pivot point but it shouldn't be, its on the same vert on both objects.