Femmed seeds ARE NOT ALL female. If the seeds have been femmed, the odds of the plant growing into a female plant are much greater, but there is no way of guaranteeing they will all be female unless they are genetically engineered, which would make them very expensive. Femmed, however, does increase your chances of growing females.
From my understanding, femming seeds has something to do with their cultivation during the flowering stages of the mother and what nutrients she receives. Please correct me if I'm wrong.