If you want to get really technical about it.. The pinion has 11 teeth, and the ring gear has 39 teeth.
39 divided by 11 is 3.545454545 repeated forever.
This is why it works having a 3.9 front diff and 1.1 center diff, you get 3.9/1.1, multiply the top and bottom by 10 and you get 39/11, the exact ratio of the rear diff.
It still doesn't make sense why subaru does it.