I think there are a few things going on here. First of all, the response to configure was a bit slow from signature and production reservations alike. Tesla first opened up sig orders and gave them a pretty lengthy head start in the Design Studio before opening up production orders. Once an order is submitted to the system, it is placed in the queue based on time of confirmation, configuration choices, etc.. The fact that it's a Signature, Founders or Production series vehicle no longer takes precedent, those are just badges on the car and different VIN # selection.
The next issue is all the early production issues. There are numerous reports that there are, and have been, quite a few X's sitting at the factory that are not moving. I'm betting there are a lot of early production signature models that didn't pass QC inspection and they are caught in limbo until appropriate fixes can be applied to these vehicles. It might make sense from a manufacturing and delivery perspective, but it really sucks for a lot of early signature orders who don't know where there cards may be at. IMO, if this is indeed the case, Tesla should probably move forward by remanufacturing new cars for these deliveries and then when the opportunity arises to repair or address those cars that are sitting, they can be used as showroom demos and loaner vehicles.
I also suspect that production series cars will also have trickled down into the queue amongst Sigs depending on configuration options or how fast a car can be shipped. There is a lot more new revenue tied to each production order vs. the handful of sig orders that are lost in limbo.
On top of that, we have component or supplier issues. Some are known like the black Onyx wheels, which really are not a factor in this Sig X discussion, but representative of the slowness of this process.
