Ok folks, I found the answer myself,
the autoPowerSpectrum is single-sided. This means that it returns an array that stops at SampleRate/2. Althought the FFT returns a two-sided complex array, when canceling the second half to switch to single-sided, all values have to be multiplied by 2.
I didn't quite notice because I was looking at the RMS value of the returned spectrum.
Bye folks