Why are there two possible arcs for the path of MH370?

In all of the reports and images of where MH370 could be (based on Inmarsat data) eg http://www.bbc.co.uk/news/world-asia-26503141, they show two possible (and opposite in direction) arcs where MH370 could be. How come this is the case? Must be something to do with which regions and areas different satellites collect data from?