A feminized plant grows from seed that has been made using pollen collected using a few methods like STS. Sure you know this. That means that the plant that you grew was female. The one you found seeds in was feminised. Unless you use feminized pollen (which you generally have to make) the pollen is from a male and that gives you regular seeds. So yes the mother plant with the seeds is feminized but she still produces seeds of both sex because the pollen isn't made intentionally to provide feminized seeds. You would not be asking about the seeds if you went to the effort to make feminized pollen. Hence its either from hermie pollen or pollen from a male that got into your grow space.