It seems like when run on SCSS mode, it expands mixins before running the redundancy check. For instance, two of my selectors both include three of the same mixins, which end up expanding into 23 rules, and CSSCSS reports 25 shared rules between them. (Further inspection confirms that exactly 2 normal rules are shared.)

I would think that using mixins should also count as having eliminated redundancy. A simple solution would be to ignore them, or more ideally the redundancy check could treat mixins just as a normal rule, so that it could detect using the same set of mixins in multiple places as redundancy.