For HKG-SVO US$200 difference round trip is also the norm for early booking in peak season - slightly larger for non-peak when availability on economy is larger - on this route I've never seen US$600 difference.
Aeroflot is famous for offering premium economy for a price less than others' economy - an Aeroflot 150% P.E. ticket is even cheaper than Finnair 50% economy ticket for the same destinations between Hong Kong and northern Europe when Finnair is operating at peak season price.