Well, if the past is prologue, then Tesla is going to initially deliver only a high-end configuration. Those who reserved this configuration will get tapped first, leapfrogging even those who may have reserved a long time ago, but with different configurations.
Additionally, West Coast reservations will likely get fulfilled first, so earlier reservations from other geographies may end up getting their cars later than those who may have reserved later, but are located closer to the factory.
So, the fact that someone who reserved recently is already getting an invitation to order is not necessarily indicative of a paucity of overall reservations.