Ian W
The Doppler tracing and tracking carried out by INMARSAT on the SATCOM signals plus validation testing against other aircraft would put the aircraft in the area being searched. It is not possible for the aircraft to have gone North and still provided the same Doppler signals. These calculations have also been checked by a considerable number of mathematicians.
But that is only true for an aircraft which after last radar contact to the northwest turned south and flew then with constant altitude, speed and track , which are the assumptions and led to the present search area.
An aircraft maneuvering in altitude, speed and track could be anywhere on a position close to the south arc and to the north arc.
Under the above assumptions they search at the most probable positions, but those are not exclusive and might be off by thousands of miles.