Sounds about right, just look at Dark Harvest.
Based on the advanced tooltip, "Damaging a Champion below 50% health deals adaptive damage". This should mean that ANY damage that a champion takes while below 50% HP should activate Dark Harvest.
But in reality, it procs on Fiddlestick's E but not Sivir's W and also procs on item passives (Luden's Echo) but not item passive DoT (Liandries Torment).
It feels like this game has very little consistency in its game mechanics and so many "gotchas".
edit: Correction, it does work on Brand passive but inconsistencies still apply to champs like Sivir.
Luden's echo proc damage is (for some reason) scripted to behave like a spell. Thus, it procs DH. If it wasn't scripted as a spell, it would not. I'm addition, it would also not apply Liandry's burn, Rylai's slow etc, which (while consistent and IMO the correct call) has significant knock ons for many champs if we changed that right now.
I agree the lack of consistently is annoying.