AH!! that is because you don't have them on the direct flight from BKK to LAX!
They are flying on two separate flights, with a 9 hour layover in seoul!
If they were on the direct through flight then it would be the 777-300 they whole way.
ATM you have them overnighting in BKK for 17 hours, then another 9 hours in Seoul.
On the way back you have them on the through service - that will only have a short transit in Seoul and will be the same plane the whole way.
preferably you want them on the TG692 out of BKK.
Do they have the requisite visas if they want to leave BKK and ICN airports?