Religion or religious history can account for at least some of the difference.
A lot of religions explicitly teach and emphasize forgiveness, even for grave injustices . When people have an attitude that says "forgiveness is required", this can have a lasting and widespread influence. Nations with a strong history in such teachings, even if they are not particularly religious now, may retain that attitude toward forgiveness.
 A friend of mine wrote, after his sister was murdered, that he had no option but to forgive, that regardless of his emotions, his theology required it. http://darrow.faithweb.com/murder.htm