BMW 手排油耗 318d Sedan better than 118d Coupe?

經濟部能源局的油耗測試裡, 四門 318d Sedan 比兩門 118d Coupe 省油 (皆為六速手排)

Car = 市區/高速/平均 km/l
118d Coupe = 15.57 / 21.35 / 18.8
318d Sedan = 18.68 / 25.73 / 22.6

My assumption is that 118d coupe shares the same engine and same transmission (manual) with 318d. Meanwhile, 118d is also lighter [車子重量 318d (1571 kg) > 118d (1521 kg)].

Assuming the data is correct, what trick does BMW have up its sleeve to make an older and heavier 318d more fuel-efficient?

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http://web3.moeaboe.gov.tw/ECW/populace/content/wfrmStatistics.aspx?type=6&menu_id=1304

進口小客車車型耗能證明101年10月核發資料
BMW (註一) 318D SEDAN (德國) M6 4D 1995.0 1571.0 9.9 18.68 25.73 22.6 汎德公司101/10/05 1級


進口小客車車型耗能證明102年7月核發資料
BMW (註一) 118D COUPE M6 2D 1995.0 1521.0 9.9 15.57 21.35 18.8 汎德公司
102/07/17 1級

註：此車型係使用柴油，其油耗結果依據歐盟1999/100/EC指令及其後續修正指令之測試方法執行。