check this topic. Looks like problem is that when you equip 2 part armor (doom hvy, fp hvy, bw light etc)the p def formula count u equip only top part of armor not both. In fact 2 part armors should get nerf compared to current state and lose these "18 p def" or w/e
https://l2classic.club/forum/topic/17595-1-part-main-armor-p-def-bug/
Ez to test is BW hvy vs doom hvy both should result in same p def in stats