I am trying write up a guide and some items with different luck stats appear in it.
People explained it like this to me. The game makes a Roll (like a dice roll) from 1 to 100.000.
If you wear a helmet with 3.9% luck it modifies now the Roll and increase the outcome of 3.9%
Then there are other luck items like food buffs that increase luck by 2.900. These come flat on top on the roll. If you now reach a certain threshold like 80.000 you get the rare item.
Because % for named drops in dungeons is high, you will not notice difference on single run. On 20 runs you will see difference. Anyway is not worth to wear luck gear if you only want get named items…