The reduction isn't a simple -x yalms. On larger knockbacks the distance reduced is larger. Implying a % reduction. But on small enough knockback it prevents it entirely. Something that shouldn't occur using a %- unless flooring is involved. Which appears to be the case.
@Pantafernando: Given the wording and personal experience, it appears that the distance reduced could simply be in yalms, hence why the long knock-backs still work and the shorter ones can be negated.