Regarding Dissection Expert and loot generation

Most of the questions I see related to this are very old, which makes me think there might be some outdated info or changes to the mechanics.

Is the loot you get from a creature generated upon creature spawn? If so, what’s the point of Dissection Expert?

I understand that Dissection Expert “Increases the drop-chance of loot from Creatures by 10%”. However, if whatever loot I’ll get is predetermined when the creature spawns, what’s the difference?

I ask this because I’ve been save scumming a Black Widow trying to get Super Spider Venom, and I always get the exact same loot: 2 Spider Chunks and 1 Black Widow fang. Whether I equip Dissection Expert or not before looting, I always get the same loot.

What’s the error in my logic here? Thanks in advance!