I'm not sure there is a catch, what you notice is rather tied to cumulative % chance and the fact that "small" units usually just die fast
.
For example, a 100hp unit that fight once, get two 10 hp wounds, then fight again and take 3 hits, only has a (100%-10%)^5= 59% chance of *not* having got any affliction !
OTOH a 10 hp unit usually either is wounded once - and often get an affliction also - then die, so you just don't care..
Overall I agree that affliction is "overdone" IMHO, and units get too much of them and/or too grievous ones - a 10 hp hit on a 100 hp creature just *shouldn't* have a 10% chance of chopping a arm or an eye out, the system should be non-linear, for example
affliction % = (hp taken/total hp) SQUARED
(that'll give 1% chance for a 10% hp^loss, 4% for 20%, up to 81% for 90%)
The other problem is that afflictions never heal except under special circumstances, and that *hit big* effectiveness of big units (except for Arco ..).