There aren't enough gates at A for all the COEX flights, so some will arrive in/depart from C. Also, if a COEX flight is arriving from a Canadian airport without US customs, it will have to arrive at C. There's no good way to predict. For my last two outbound CO-mainline/return COEX itinereraries, I parked at A and took the shuttle to C, thinking it would be a quicker exit from the airport on the return. Both COEX flights arrived at C.