The FE gets better efficiency ratings for two reasons. The lack of roof rails is the real efficiency gain. The other one is that it weighs slightly less than the LX model. The EPA has weight groups and even shedding one pound that puts you in a lower bracket improves your mpg rating. That accounts for the extreme weight reduction steps Hyundai did in the US market for their Ioniq Blue model to get the highest EPA ratings of any car, including Prius.
Weight certainly has an impact on fuel economy but 50 pounds doesn't matter as much as aero. I'm getting considerably above the EPA rating for the FE, never mind my model LX rating. Of course, driving style plays a large role. I also removed some weight - about 45 pounds net from the rails and rear seats after building a new load floor. More importantly, one more aero mod: removed passenger side outside mirror.
2018 Kia LX HEV Metal Stream with Advanced Tech