Tesla don't build to order, so all orders, whether custom or from inventory end up being matched against inventory. The question really is the timing and do tesla sometimes, maybe accidently, publicly list an inventory car when there's an outstanding custom order of the same spec?
Maybe a custom order has been matched to a later delivery (say Dec as that was the first available car when the matching occurred) and then a cancelled order comes in for a car due to be delivered in Sept. Tesla don't change the order matched for December delivery, they just make the cancelled/sept order car available on Inventory and you need to buy it and change the order yourself. It would make sense as there are probably hundreds of outstanding orders for those spec cars so who do you pick?
The evidence seems to suggest at least one of those things is happening.