Thanks for all answers,
Vilas & Cough, what I understand, your explanations assume manual/modualting brakings. However, this does not explain why we are adding this OW Landing Procedure extra distance to AUTOBRAKE LOW/MEDIUM setting, because these setting are already assures a certain decelartion rate (at least for dry runway).
By now, I couldn`t find any explanation in offical docs like FCOM&QRH; just some Airbus presentations which are not offical, yet do not mention how to use OW factor.
Another Wonderful(!) Airbus Logic as Nitpicker mentions: For the Landing Distance with Failure cases, there is not such an overweight extra distance factors, though that distances assume Max Manual Brakings.
Wonderful (ill-)Logic.