Necromancer Nightwalker passive might be bugged (or is it a hidden feature?).
Poison length reduction as the skill states (at lvl 15): 60%. Char sheet reads: 168%.
I've checked my inventory and there's literally zero charms or relics on me with poison length reduction on it.
BUT: I've got +9 to all and +18 to class skills, and removing one skill level reduces the value by 4%.
so (9+18)*4% = 108%; + 60% = 168%, voila, hidden slvl bonus.