Interesting. Tesla uses the same width tires (245mm) whether with 19" or 21" wheels, and its EPA rating does not change with wheel diameter.
The Lucid that was tested at Windshear wore 245mm tires on 19" aero wheels. The 21" wheels we've seen on recent Dream Edition pictures are also aero wheels. Unless Lucid uses a wider tire on the 21" wheel, the difference between 503 and 517 miles would seem to relate to something else.
The overall diameter of the 19" and 21" wheels is the same. I would not expect any difference in the EPA rating.
I would think that the difference between the 503 and 517 miles might have something to do with included equipment draw or weight difference?