Why is there a flight time difference between a flight from Los Angeles to Seoul Korea and Seoul Korea to Los Angeles?

Prevailing winds at altitude.

This is similar to running against the win vs running with the wind.

Flights:

Seoul (SEL) to Los Angeles, CA (LAX)

Flight Duration 10 hours 40 mins

Los Angeles, CA (LAX) to Seoul (SEL)

Flight Duration 13 hours 10 mins