Sounds like the viscuous coupling is allowing the rear wheels to "over-ride" the front ones just a tad. If you've ever driven a 4WD vehicle on dry roads this would be most familiar to you, as the rear diff, with no slippage, runs forward, pushing through the front tire/wheel which is at an angle, and moving forward at lower relative speed. This is never noticed in mud and snow with the road surface allowing things(tire contact) to slip 'n slide. The viscuous coupling(on MT cars) and the 4EAT (on autos) allows some loss of forward momentum at the rear(effectively lowering the gear ratio) when this occurs in our AWD cars, thus allowing efficient use of AWD on dry roads. Hope that makes some sense.
Something you may wish to discuss with a Subie tech to determine the degree of severity and whether or not it's really a problem.