Here is the simplest answer. The "AWD" system in all Subarus is realy talking about the viscous coupling center diff. This transfers power front to rear. The limited slip diff (LSD) in the rear of 2000 and higher 2.5RS's and WRX's transfers power left to right at the rear of the car. The front differential is an open type. Meaning that one tire can be stopped and the other tire can spin. But this would only happen in a few very special cases (like one tire jammed between two boulders and the other tire in the air). In normal use power is divided equally between all four wheels.
So yes in some cases your Subaru only has three wheel drive. Just like your '72 Duster that had an open diff in the rear would only spin one tire. But does that make the car one wheel drive? No, it just means that only one tire gets the vast majority of the power when traction is broken.
-edit- wow check out the times of the last four responses, we all started typing baically the same answer at the same time.