Vesper Set

Hello i have ask why vesper set don t give more p def when those are +6 or hinger it same it is +0 or +6 same p def almost "When all set items are enchanted 6 or higher, p def and m def will be incressed" and p def and m def not change when item is +5 or +6 .. :p i m not sure do im right or not, but i wana know and one think more do vesper or mordor weapons got any difference ? sry for bad english