To emphasize, there is no longer a difference between US, EU, or "Asian" ratings for down used in garments as tested by worldwide independent labs for international sale. They all follow the IDFB (International Down and Feather Bureau) "Fill Power" standard of measurement and rating, e.g. sample inspection for down content and quality, steam preconditioning and Lorch cylinder test. It has been this way for several years now.
Of course, variation in test accuracy between labs around the world can be expected as well as the factors alluded to, e.g. fill power changes over time with humidity, temp and age/wear & tear on the garment.
Over time, lower FP down fares better and is more cost effective and the preferred choice when weight and volume are not factors, e.g. bedding duvet. But for UL backpacking, high FP is the desired primary feature. With consumer pressure on for ethical supply chain, high FP down is getting more expensive, especially in the last year, and may be why we are starting to see down/synth blends coming to market.