Retrieving an arrow that you have shot adds two arrows to your inventory. This "two for one" ratio stays constant, it doesn't increase geometrically as suggested by the original report. i.e. start with a bow and 1 arrow. Shoot the arrow, retrieve it, note there are 2 arrows in your inventory when there should only be 1. Shoot 1, retrive it, you'll now have 3 arrows (one remaining in inventory + 2 from the one you just retrieved)
I shot an arrow at my wall and when I picked it up it became 2 then 4 and so on.