There are a whole host of reasons why two cars could return different times:

Weather conditions, location, weight of driver & passenger, fuel load, tyre condition, whether the car has been chipped etc.

You can have different differential ratios fitted to improve your acceleration at the expense of fuel economy and top speed. Who's to say that the SLK was running the standard.

Since the RX-8, TT, 350z etc. all have the performance you would expect from their figures, then I would think that a nominal 19BHP would not account for that difference in quoted performance.