If that "since 2013" bit is true this may not be relevant, but Ars Technica just had a piece on how the slow ramp-up of production on the part of Tesla has had the effect of extending the full $7,500 federal EV credit through the end of 2018, and there's been speculation that Tesla was intentionally delaying delivery of vehicles to get as many customers as possible the full $7,500 federal EV credit.
https://arstechnica.com/cars/2018/07/tesla-sold-200000-cars-in-the-us-so-the-7500-tax-credit-is-going-away/