Question to DUE Timer library- Frequency

Official Hardware Mega Due
1

Hi there

Im using the DUE timer library to count an Event with a certain frequency in (Hz) from 0-100 Hz

I figured out that my frequency output is true till about 40 Hz and then I dont get the right frequency

I used the DueTimer.h Library

Can anyone tell me where is the bug in my code ?
I controlled my output frequncies with an oscilscope

#include <DueTimer.h>// Arduino DUE timer


unsigned int Frequenz=20 ;//Desiered output Frequency 


void handler()// Interruptshandler 
{ // I removed the other programm codes which are irrelevant to my question
  }

void setup() {
  // put your setup code here, to run once:
  
  Timer4.attachInterrupt(handler);
  Timer4.setFrequency(Frequenz*66);// Here is where I set a certian Frequency as output
  Timer4.start();
}



void loop() {
  // put your main code here, to run repeatedly:

}