It's all about which plastic they have in the machines, which molds are available, how many of that mold they have (and you have to consider row profile, legend, as well as size in that), materials availability, transfer sheets for the dye-subs, and so on. The logistics actually get pretty complex, and I'm sure their system addresses their restrictions fairly well, in spite of the maligning SP gets in the forums when there's a delay or the queue for an order is 6 months long.