Shrimad Ramayana provided the reason for why 14 years; becomes one generation if 14 years are completed. Those who have been associated with Perumal shall not accept anyone but Him, but after 14 years the next generation begins and people from that generation could associated with Bharata, and they shall not accept Rama. If Bharata rules for 14 years, he can remain on the throne continuously. This was the reason and nothing else and there is no mention of any other set of years in Ramayana.