Its that way on my (previous generation) 03 Outback, too. Don't know if it's been that way since MY00. There's some extra defroster-type lines on the rear window. Wagon owners may have those lines on the rear side fixed glass, too- I've seen other wagons & some SUVs do it that way.
I think there's technically more than 1 antenna, & the "diversity antenna" feature lets the radio dynamically use the best receiving one.