My personal guess the Model S Plaid is the fastest accelerating production car currently and you pay a premium for that, the Model X Plaid on the other hand isn’t. S better handling and will have track mode/top speed of 200mph at some point.
The Plaid+ got dumped leaving the Model S Plaid to be the fastest, with the dumping a 10K increase immediately happened.
If I had to bet the Model S Plaid is cheaper to produce. Less metal, no falcon door, and many other aspects.