He is just guessing, as he does all year long. What probably happened is the monthly guesses throughout the year came to a much lower number than was hinted by Elon in the yearly guidance on the last financials call, so he jacked up December's numbers.
The monthly deliveries are much less balanced due to Tesla's shipment method (US one month, Europe the next, Asia the next), and nobody actually knows what % of cars are going to the US anyway, let alone the exact # of cars produced.
As I've written before on my monthly sales reports, anyone reporting Tesla sales is guessing.