Sheep Guts
The south china sea is on the 40 degree (satellite altitude) line as you say.
However, it is not included in the final 8:11am locations (marked in red on the Malaysian photo) because that area is also covered by another INMARSAT doing ACARS coms (called POR which is over the Pacific).
If the plane had ended up there then the pings would also have been picked up by POR. Obviously they were not hence the corridors start West of south-east asia.
You can see a map of INMARSAT coverage back a few pages or just search on google.