Mr Choi,
The regulator has some schedule built in to the mask. Above a certain altitude, it supplies 100% oxygen content at 100% pressure (the same as EMER mode).
Maybe for your mask (KAL? Jeju?), the altitude where the diluter/demand is at 100%/100% is FL390. Where did you find the FL390 reference?
According to the Avox/Scott engineer, they want crews to go to EMER first when there is a problem, and then sort it out after.
So you're climbing out, MASTER CAUTION, CABIN ALT, etc, you don the mask and go to EMER. If you're still conscious after that, you can go to 100% or NORM.