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My guess: Tesla had originally planned on equipping the Performance Model 3 with a staggered set of wheels, but changed their mind at the last minute because it resulted in too much loss of range. They actually reached a point of having several staggered wheels manufactured. Somehow, people are obtaining these wheels as Tesla "throws them out".
I base this one two things:
The original alpha cars had staggered 20s.
Musk's tweet about the decision to offer a square setup for the Performance car:
The car shown in #4 would have been part of the initial "Performance test-drive fleet," whose production predated P3D customer deliveries 6 weeks. Given that timing, I wouldn't be surprised if Tesla also conducted a final round of user-acceptance testing among employees using these early-build P3Ds, and it was their experience with range that led Tesla to nix the staggered setup at the last minute. (The added manufacturing cost of making two different sets of wheels for the same configuration may have been a factor as well.)