Simple logistics.... home airlines ie Indian airlines fly direct point to point, whereas foreign airlines will take traffic via their home hubs and therefore will charge less than what home based airlines will charge.

It is the same almost anywhere else... while EK is the cheapest from say DEL to FRA via DXB, their fares ex DXB are the more expensive than from India cos its a direct flight from where they are.

Similarly with SQ. SQ is the cheapest from India to say USA via SIN and their fare is cheaper than originating from SIN cos SIN is their home base.