I bought this deck with a G16XL but it wasnt fitted so i think your guess is right.The double sheave pulley on the center spindle is not correct. I would guess that someone either tried to convert this deck to fit on an XL series, or they got the wrong replacement pulley. Everything else looks correct, although it is not the best picture. The PTO belt from the tractor runs to the rear pulley assembly.
I don't have a good scan of the correct belt routing, but the picture I've attached is how it should be.
the bottom middle pulley has no beltNot knowing anything about the deck I would say it looks correct
Why you recon its wrong??
Think pully maybe for a sweeper or blower