[lunarhighway]

newer cars tend to be much taller than older ones, this allows for a more upright seating position, wich eliminates the need for more legroom and thus allows for more usefull cargo space or a relative shorter vehicle although whitin the same model family sizes tend to increase.

this has the disadvantage that while drag coeficients might have slightly improved (although this is not always the case)
frontal area has increased quite a lot withing the same model range over the years giveing more overall drag.

also weight increased because of more and more safety and comfort addons some of wich take a lot of energy wich has to be supplied by the engine, creating aditional electric and mechanical load ,direct and indirect cooling requirements.

this would explain why newer models have worse FE than older ones, but not why a wagon could have better FE than a sedan.
even if they have a comparable if not a more refined engine.

it's unlikely but not impossible that it has a slightly better drag quoeficient.

perhaps certain standard features of the sedan are an option on the wagon, tire size, climate controle, airbag configuration, etc etc, might differ between the base models, since the wagon is more practical oriented, some customers might perfer a cheaper more stripped version. this might make a difference in weight and electric load and in case of the tires frontal area and Rolling resistance... this could produce the FE difference for both "standard configurations"... it's not entirely unimaginable that carmakers produce a stripped out version as stock wich would get better FE figures on paper, and than offer cheap or even free upgrade packs wich give the cusommer the impression they're getting a bargain, wihile their actual FE goes down.

also it is possible engine management en the gearbox ratio is different, since the wagon would be expected to haul more weight than the sedan, wich might be configgered more for a more sporty driveing experience.

that's about all rational explanations i can think of