I think you might be misreading the graphs. According to the article a 1 year old iPhone depreciates by about 40%, and a 2 year old iPhone depreciates by around 65-70%.
Thus, a person selling their 1 year old iPhone would fail to recoup 40% of the sticker price overall while someone buying 1 year old phones and selling them a year later would fail to recoup all but 25% of the sticker price, give or take.
Thus, a person selling their 1 year old iPhone would fail to recoup 40% of the sticker price overall while someone buying 1 year old phones and selling them a year later would fail to recoup all but 25% of the sticker price, give or take.