A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 100,413
It might be fare class availability, possibly in combination with rules regarding combinations of fares and open jaws. To verify, you'd need to look carefully at the complete fare rules. The $5404 fare you're seeing must be FULL Y or something close.
Is the ICN-SFO operated by KE available as a DL codeshare? How about SHE-ICN-SFO?
What happens if you try the experiment of doing a dummy booking for something like SFO-ICN-SHE RT first using your travel dates and then maybe changing each date a bit in each direction to explore fare class availability?