If you ate the same type of livestock in an Asian country vs the US, you will notice the meat is more lean and less fatty. I assume they don't use hormones as much as the US but I could be wrong.
Body builders eat white meat chicken as the source of lean protein with the least amount of fat content so perhaps the same concept works for fish. Prepared fish pellets has lots of filler while this is straight up protein.
What aobut these fish foods that state quick growth vs those that doesn't. Is it just a marketing ploy. I feed a bottle of Hai Feng fish food, i've used it and seen some good results in growth with my kois. I've traded them since for smaller ones and ran out and the new ones don't grow as fast on Hikari staple koi food.