Well, it's cheaper.
I thought the whole reason they thought they switched to the smaller filter was due to a clearance issue(for heat) on the turbo blocks. They have never come out and said it needs replacement.
I'd check the 05 RS service manual and see what it states is the Subaru replacement part for the EJ25 NA engine. If they still allow the AA060, and it's cheaper, that would be the logical choice.
Maybe they just use the AA100 on the new cars to simplify parts availability on the line.