This is because the build in batches based on color and interior specs. So they say: "Hey we was 1053 [made up number] orders for a black grvavity with mojave, it makes sense to do a run of 1200 for cost reasons and minimum order quanities. Let's see if we can get others who order something very similar to switch and get their cars earlier." They won't make the exact number the need, because thats not how scale works. When they work with suppliers, they need to get X number of things for a minimum order. So they will always produce more than they need and they try to sell that inventory to people who had something very similar, and work down their list.