Thanks jimjim and stormy
It is very clear to me from the photo of the Malaysian SAR map
https://pbs.twimg.com/media/BiwPWMOCYAAG3ZC.jpg:large
cross referenced with the INMARSAT coverage map
http://www.inmarsat.com/wp-content/u...ember-2013.jpg
that they are doing the calculations on the basis of of only one INMARSAT being in contact with the plane (at the end).
Only the Indian Ocean (64E) INMARSAT was involved in the last ping(s) and so they have ruled out areas (on the 40 degree circle) that overlap with coverage by POR in the Pacific and AOR-E over the Atlantic to the West.