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The comments that wagons have higher drag because of the larger wake area assumes that in the equivalent sedan the airflow separates at the rear of the trunk lid. We all know that this isn't necessarily the case and lots of sedans can have flow separate at the rear of the roof, giving you a wake area equally as large as a wagon plus a ton of rear lift. The airflow on a wagon more than likely stays attached all the way to the rear of the roof, negating some rear lift but still giving you the large wake. Since the wake vortices alternate (ever feel that side-to-side buffeting in the draft of a tractor trailer?) a wagon has one pair of vortices alternating behind the hatch, where a sedan could have a pair of vortices behind the backlight and another pair behind the trunk that interfere with one another and increase the pressure drag.
Pure speculation of course, but it just goes to show that the article goes into gross generalizations and the 2003 Jettas appear to be an exception to the generalization.
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