Your "Superuser Security Types" for that rule index is set to "Any", so the "Root" output is correct based on how that portion of the rule is set up currently. However, it does look like the output is incorrect when the rwrule setting is set to "never" for some reason. In my lab, when I duplicated your policy but set the rwrule to "none", it returned a null value for the "ReadWrite" property.
I tested it in the actual ZAPI call too and it is incorrect there as well. Looks like a bug of some sort.