Inspired by this question on World Building, I'm curious if the bonus stacking rules from various editions of D&D have ever had an in-world justification published.
From what I've found, there's basically been two types of rules:
- Bonuses of the same type don't stack (except for some edition-specific named exceptions): D&D 3e,3.5,4e; PF 1e,2e
- Everything stacks, unless the source specifically specifies otherwise (and many things specify otherwise): OD&D, AD&D, D&D 5e
There's generally good mechanical reasons for the first kind, and thematic reasons for the second, but I've never actually seen an in-world justification for any of it. Has any official rules source (not author commentary) ever explained what characters think about why bracers of defense/armor don't stack with mundane armor, but a ring of protection does? Or why three different spells have overlapping but not stacking effects? Or do they just say "it's magic, it doesn't have to make sense"?
In lieu of an official rules publication addressing it, an answer from official D&D-based fiction (such as the Forgotten Realms or Dragonlance novel series) could also work here. Since those are written from an in-world viewpoint, it would be an example of how at least one character understood it. (It's plausible that the in-world justification is different across different settings, but I would be surprised if it came up enough that there'd actually be a difference.)