Without paraphrasing the entire QRH notes section: The Normal Configuration Landing Distance is already factored and is advisory for different runway surfaces and weights, as you correctly state, ISA deviations should be added to obtain the actual landing distances required for performance purposes.
The NNC landing distances relate to aircraft non-normal configuration and are based on configuration and "speed" hence the corrections for ISA deviations are not required.
Last edited by Avenger; 8th Feb 2011 at 13:36.