Zeny Knife, like *all* items that lets you get something from killing monsters, have a decrease in activation rate. It was 2% in the past I believe. Now it's 0.4%.
A lot of items have the wrong item description, but not much reasoning to fix those ASAP since eA will naturally add them as time goes on. The most influential one is probably Ledger of Death, the Beelzebub drop. It is NOT 30% coma rate on demihuman, but 10%. The script for the item properly reflects the 10% activation rate.