I was puzzled as to why the remainder of the 40 degree circle was disregarded. I thought there might be an attempt to misdirect our attention away from Africa for whatever reason, but the explanation is much simpler. Here's the apparent answer:
The Malaysian map shows that the SAR team had concluded that the last ping came from somewhere on this 40 degree circle but they ruled out parts of the circle in the far East and also most of the west half of the circle.

A examination of the coverage of the INMARSATs explains this. The last ping must have been picked up by IOR over the Indian Ocean but not by POR over the Pacific or AOR-E over the Atlantic. Hence bits of the red circle are not valid and we end up with the two arcs, also described by the PM as coridoors.
Effectively, had MH370 been at any other point on that circle, it would have been picked up by at least one other satellite.