Wilford Woodruff teaches that not everything Joseph taught in private was later made public.
Wilford Woodruff, "Necessity of Adhering to the Priesthood in Preference to Science and Art," Journal of Discourses, 26 vols. (Liverpool: Asa Calkin, 1858), 5:83-84
I have seen men in the days of Joseph bring up principles, and read, and teach, and advocate theories, when the Prophet would say, "It is not right to do so: they are not true." Those men would still argue, maintain their position, and they would write in defence of their theories when the Prophet condemned them, and they would say, "We have no faith in your theory, nor in the system you present." The very moment a man does that, he crosses the path of the servant of God who is set to lead the way to life and salvation. This is one thing that the Elders should carefully avoid. The fact is, there are a great many things taught in the building up of this kingdom which seem strange to us, being contrary to our traditions, and are calculated to try men. Brother Joseph used a great many methods of testing the integrity of men; and he taught, a great many things which, in consequence of tradition, required prayer, faith, and a testimony from the Lord, before they could be believed by many of the Saints. His mind was opened by the visions of the Almighty, and the Lord taught him many things by vision and revelation that were never taught publicly in his days; for the people could not bear the flood of intelligence which God poured into his mind.
How was it in that day in reference to many things that were taught and practised? All was not revealed at once, but the Lord showed the Prophet a principle, and the people acted upon it according to the light which they had. All the perfection and glory of it was not revealed at first; but, as fast as it was revealed, the people endeavoured to obey.