They are using the VIN as a placeholder as a way of allocating an in transit car to a specific order without having the actual VIN at the time. You’ll find the placeholder VIN will be replaced by the real one when they have it. This is probably why VIN numbers disappear temporarily ( during that change ) .
I suspect the reason some people see VINs assigned and then not replaced is because Tesla are adding some buffer to the number they assign in order to account for the possibility of order cancellations etc. So there may be times where , say, 110 people are allocated a VIN but there are 100 cars available and all 110 people still want the car at the point the real VINs are allocated which means 10 people lose out and have to wait for the next batch.
Maybe...