The stock body is made to fit on either a carbine tube or a standard rifle tube.
When used on a carbine tube:
The stock comes with a metal pin that inserts through the bottom of the rifle stock and engages one of the holes on the carbine tube. The pin can be removed and the stock moved aft, as long as the hole in the stock lines up with one of the holes in the tube. You are correct, any adjustments make it longer not shorter.
The A1 length stock, when used on a standard carbine tube will be A2 length or longer. This is because the stock cannot mate flush with the lower receiver like it does on a standard rifle tube. The stock is setback to clear the castle nut, and as a result the A1 length is now longer.
If this is the case then why do it? Well the rifle stock offers a greater cheek weld then the carbine stock, and it is easy to swap the two out. So in theory, you could use the rifle stock for precision shooting, and the carbine stock when a shorter profile is required. And the advantage here is you can run either stock on the rifle without swapping out the carbine tube.
This may not be of any benefit to you, but to those who are issued a rifle that they cannot make changes to, military, police, etc., or have a rifle where the carbine tube is staked in place and not removable, this gives them options they wouldn't have otherwise.