The reason some find differences between bulk and tin is mostly because the bulk is stored in plastic bags, which are permeable to air even if the big isn't opened much. The more you open it, the greater exposure to air, and the chances of diluting the strength of the toppings (if any exist) and the tobacco itself. Also, once a tobacco is tinned, it starts aging immediately. That doesn't happen with bulk.

The reason FVF, SJF, etc., are cut differently for tins is obvious: to make them fit.