Why Can't the Width level of the Window Trigger on the Digital Oscilloscope DL1500 Series be set to Below the Voltage Sensitivity?

The width level of the Window trigger cannot be set to a value less than the voltage corresponding to 1 division at the specified voltage sensitivity. For example, if the voltage sensitivity is set to 500 V/div, then the width level cannot be set below 500 V. This limitation is due to the fact that the trigger sensitivity of the Digital Oscilloscope DL1500 Series is 1 Divp-p.