Tone generator irregular pulse frequency, and pulse width

Hello, I just trying simple pulse generator with tone(). But when I watching pulses on oscilloscope I see some pulse tremor and irregular frequency. I would like to asking somebody to help solve problem. There is link to youtube to see what is going on:

https://www.youtube.com/watch?v=PH_dPQtzA8A

void setup()
{

pinMode(5,OUTPUT);
}

void loop() {                 // put your main code here, to run repeatedly:
tone(5,20000);

}