Are you the original owner of the car? Perhaps the previous owner had the ECU tuned and altered the rev limit.

And MJ: I totally agree with you on the tiptronic. Apparently Mercedes doesn't want the driver to decide when the car shifts. This annoys the crap out of me too. You tell the car to shift and it shifts when it feels like it. You don't tell it to shift, and it will shift when it feels like it anyway. Grrr.