Why can't I add a hoodie to my design?

Certain products have certain print area restrictions. A design that fits on a t shirt won't necessarily fit on a hoodie, however whatever you can fit on a hoodie, will fit on a t shirt. 

If you start your design on the hoodie, you can add a t shirt for this reason, but if you start on a t shirt, you can't add a hoodie. The image below fits on a t shirt, but as you can see it won't fit correctly on the hoodie.