I don't think it matters that much. It shouldn't matter how long they are stored frozen; what matters is how long and what conditions it has been after that storage period. I don't see how you could ever get straight answers from the vendor(s), so I think it's a crap shoot, when you buy.
But logically, I think you are right, that your winter is the best time for your shipments.
-- Bulbs we buy in our winter/spring are dug in the previous late summer/fall and pretreated.
-- Bulbs available in our early summer/summer are just leftovers from these same pretreated bulbs dug in the previous late summer/fall.
-- Bulbs we buy in our fall are the freshest, dug within the last zero to two months and not pretreated.
So theoretically, the freshest pretreated bulbs are available for our late winter/spring shipment, and your fall/winter shipment. Add to that extra shipping time, and you get winter shipment for the southern hemisphere.
