I'm not saying this is why we get weird combos of power train, differentials, etc, but I stated this earlier and I think it's a good reminder. The regs for CAFE standards and GHG emissions require very complicated calculations. Here's a sample in layman's terms from a KIA of America Regulatory Compliance Manager position job description:
The Senior Regulatory Compliance Manager is principally responsible for definition and management of Kia’s CAFÉ and GHG compliance strategies, and formulating related recommended product actions (powertrain, volume/mix, distribution) necessary to comply. This includes monitoring fuel economy and emissions regulations and related legislation, and Kia compliance with CAFÉ, GHG, and as needed, ZEV regulations, as well as performing related calculations/analysis.
Define, recommend, and oversee Kia GHG (incl. Tier 4) and CAFÉ compliance strategies, and as needed, ZEV strategies, as it relates to powertrain recommendations (type, mix%, timing) and distribution (region/state).