When you attempt to get the value of an unchecked check box with JavaScript, it returns the incorrect value. Instead of returning the string "Off", it returns a zero-length string.
For example, the following calculation script for a text field will set the field to blank when the check box (cb1) is not selected:
// Custom Calculate script
event.value = getField("cb1").value;
It returns the correct value, which is the export value, when the check box is selected.