There is no explicit mention in the OP of the 20% discount.
There is a 10% discount for most international award tickets in coach; there is a 20% discount for some international award tickets in coach, and a 25% discount for most international award tickets in business class.
The 20% economy discount applies only to itineraries including the following flights:
.... Delta-operated flights from/to ATL to/from AMS, ARN, BCN, BRU, CPH, GYE, KIN, KWI, LGW, MAD, MAO, MGA, MXP, NRT, PLS, PRG, TGU, UIO, ZRH; from/to CVG to/from AMS, FRA, LGW; from/to JFK to/from BOG, BRU, EDI, GRU, LHR, MXP, PSA, VLC, ZRH; from/to BDA to/from BOS, LGA; and from/to PIT to/from CDG and on NW/KLM operated flight from/to PDX to/from NRT, PEK, HKG, SIN; from/to SFO to/from NRT, PEK, HKG, SIN; from/to DTW to/from NRT, HKG, PUS, FRA, LHR; and from/to MSP to/from SIN, PEK.
That 20% discount on select international itineraries brings economy class award pricing down from a standard 60k miles for US-Europe-US award itinerary to 48k miles if "low" inventory is available.
For the majority of the DL promotional period, this discount makes DL-issued awards in coach slightly cheaper in miles than what UA, US and CO normally charge but still more expensive than what AA normally charges in miles.