Why is that? Well, possibly due to the 991 Turbo not yet being on the market so the next best thing is the 2013 GTR. The standard SLC is supposed to go up against the Carrera S though so this may be the SLC 'S' version testing.
Newest reports state the SLC base model will be a 485 horsepower 4.0 liter twin turbo V8 code named M177. This same motor will then be upped 505 horsepower for the S model. The previous 585 horsepower rumors seem to point to that output for the Black Series model. The Black Series of course being aimed then at the 991 GT2.
What is certain is Mercedes AMG is very serious about this vehicles performance. And that is something BenzBoost respects.