At 600 kHz the minimum pulse duration is 0.833 microseconds, too small to be measured with pulseIn() which measures in full microseconds.
At 20% scale the minimum pulse duration is 4.166 microseconds, a reasonable minimum for pulseIn(). This also has the effect of averaging 5 readings to reduce noise.
The 2% scale averages 50 readings so it is even more immune to noise.