I think they should at least keep the U.S. on par with other countries-- considering we are already seeing deliveries in Germany, UK, and Canada of 00:16 and 00:17+
But I assume they're doing it by percentage per Country. Which, unfortunately, would make the most sense.
Surely what would make the most sense is fulfilling orders in the order they were placed, according to availability. There should be enough orders in each country to make that viable without significantly increasing shipping/logistic costs.