So, in a typical Mfg environment, you would utilize an ERP system to track cost/movement of the product you are building(Tesla developed their own system). Typical ERP systems use "Work Orders" to capture parts going into the product, along with a physical traveller packet that moves with the product being built. Tesla cars are individually serialized, so each car will have it's own "Work Order" where you would issue the required parts to capture the cost. If an LED Headlight is out of stock, the car could still move out of the factory, but the Work Order could still stay open under "Work In Progress" status in a "Service Center" inventory location, with an exception note "Operation out of sequence" within that Work Order/Traveller. As long as it stays in "WIP", the material/labor cost does not get transacted into "Finished Goods" bucket within accounting, which is when you would consider the car "Built".
There are ways to close the Work order short, meaning Missing parts/quantity etc, but you would not want to do that on a single unit workflow, and you would certainly not want to count it as "built", since you would have to open a rework order to back it out of "FG inventory" anyway.
My belief is, given the huge spike of deliveries at the end of 2015 and the unexpected slow ramp of X in Q4/15 & Q1/16, Tesla had to drain the pipeline and demo inventory as much as possible just to deliver what they did in Q1/16. Now that X is finally ramping up, they are using Q2 to build the transit/demo pipeline back up. Also remember, all the stores and Galleries and Service centers now ideally need twice the demo/loaner cars with 2 different models to show/demo/sell/service as opposed to just 1 model. Hence the wider gap of Built/Delivered IMHO...