My guess is its not random, just not a pattern that helps us make sense of things.
how many spec variations are there? Eg lets say they have pipelines for RWD, LR, P model 3. Then within each pipe they have separate lists for each colour, each interior spec, wheel choice.
Then for each of those lists they'll have production estimates and put some on boats. Then they'll do the matching which may be more likely by car/sku than by user.
I would expect that if you and me both ordered exactly the same spec - and that spec is on the boat now - then whoever ordered sooner should get matched first. But you can see how a different spec might get assigned to you even if you ordered later, because that can depend on what is on the boat