Its kinda the whole thing with power levels in general. Someone always has to be stronger. Even in the old canon, you have guys like Revan who were arguably stronger than Luke but never reached their full potential. It would make sense that Luke's bloodline has to be one of the strongest in the known Galaxy if we take Cade Skywalker for example, who can resurrect the dead and eventually posses mastery of both light and dark side....
Rey if all holds true, being the relative of Luke could be the next greatest Jedi.