The determination of the southern arc was made because of a doppler rate change that occurred when the satellite made a u-turn in the sky. The Inmarsat drifts north of and south of the equator in a 24 hour cycle. It forms a figure 8 against the sky. When it made one of those turns the doppler shift of MH370 changed in such a way as to determine which track it was on.
If the satellite was drifting south and MH370 was flying south, and then the satellite made a u-turn and headed north, it would be expected that MH-370 would be seen as moving away even faster.
Had MH370 been on the northern arc, it would have been seen to move more slowly away from the satellite when the satellite made the northern turn.