So while I was changing brake pads on my 2003 Outback automatic (H6 3.0, if it makes any difference), I reattached the rear driver's wheel (the car is RHD, so the driver's side is the air intake/fuel filler side) and began tightening the lug nuts. To my surprise, despite having the transmission in Park, I was able to spin the wheel while it was in the air. I had the parking brake applied while loosening the nuts, so I didn't notice until that point - I took the brake off to attempt to replace the disc, but was defeated.
On the other side, I did the same, but as expected, the transmission locked the drive shaft and I was able to loosen the lug nuts with the wheel in the air.
Given what I know about AWD vehicles, I'm pretty sure this shouldn't happen, unless while parked the transfer clutches entirely disengage the rear wheels (I know the Subaru transmission is predominately FWD, transferring to the rear wheels as required) and something else was locking the passenger side wheel. I don't know if the car has a limited-slip rear differential, but even if it did, I'm not sure it would be responsible. To re-iterate, the other 3 wheels were on the ground at this point.
Is this normal, or am I likely not getting drive to the rear driver's wheel?