This question already has an answer here:
However, the authentic cadence in natural minor, which is v-i, does not meet the definition by Kostka/Payne here. The first chord, v, doesn't really contain the leading tone (as the subtonic and the leading tone are two completely different concepts), while the second chord, i, is the tonic. In this view, it seems to be more plagal, but it isn't.
So, why is v-i in natural minor viewed as an authentic cadence while it doesn't fit the definition by Kostka/Payne?