Note: Discussing exploits is against forum rules so be mindful of that when commenting on this thread.
Item drops USED to be determined on enemy death, but that lead to a lot of people using the quick-save feature as an exploit to re-roll item drops. That made it a lot easier to get rare drops from enemies.
To prevent exploiting, item drops are determined in a different manner. To avoid, opening the door to discussing an item drop exploit, I'll refrain from giving too much details on what enemies item drops are calculated on death, which are calculated on map generation and which appear to be locked when they spawn in the world.
For simplicity, just accept the item drops are varied, with rare drops being rare.