They were not building any cars for North America in October or November so while it seems like a really long time you weren’t really advancing your position at all during those two months. When they did start building in December they build by trim and by color choice. So if the first lot of cars were all black and you ordered a blue one, you will see a lot of people taking delivery of black cars while you are still sitting around. And they always ship orders furthest out (east coast) first since the transit times are much longer.
I know it’s frustrating but it’s not random. There is a methodology to how Tesla assigns cars and they are very sensitive to making end of quarter deliveries, especially when tax breaks are concerned. Their communication could be a lot better, but they do eventually come through.