Couldn't it be the difference between merely contaminated and, essentially, "made entirely of impurity" though? The gui can't cultivate out of impurity because there's literally no part of them that isn't impure to cultivate. But if you're only contaminated, maybe you can cultivate the bits of non-contamination until they outweigh, overtake and oust the impurity.

Or maybe I don't understand anything about how cultivation works. ;-p