Then it's probably a coincidence. The calculation rounds up the result to the nearest exponent of 20, but it doesn't take anything above redline into consideration, because you're not supposed to go into the red zone. You can, thanks to the fact that the turbo disables the rev limiter. So the top speed of the upgraded Countach is probably slightly below 400 without the turbo, so once you use the turbo, it goes faster than 400.
I'll add a bit of buffer to the max rpm in the calculation. That should increase the top speed on the speedometer.