The 777 auto throttle has a lot of modes. Push buttons on the mode control panel are labeled "Auto Throttle", "Vertical Nav" and "Flight Level Change". Experts on the 777 are saying that the "Flight Level Change" mode actually pulls the throttles back to flight idle and leaves them there. And they think the Asiana crew somehow selected Flight Level Change instead of "Auto Throttle". What the Vertical Nav mode might do is unmentioned. Apparently Flight Level Change is actually a strange kind of standby, I cannot think why a pilot would use the "Flight Level Change". The situation is so bad that 777 pilots refer to the problem as the "Flight Level Change trap".
Sounds like some human factors work wasn't done right when the 777 was designed. On the other hand, the plane has been flying for 20 years accident free, so it cannot be all wrong. Certainly the crew failed to monitor airspeed during the landing.