At the same time it should be remembered that not every statement made by a Church leader, past or present, necessarily constitutes doctrine. It is commonly understood in the Church that a statement made by one leader on a single occasion often represents a personal, though well-considered, opinion, not meant to be official or binding for the whole Church. The Prophet Joseph Smith taught that “a prophet [is] a prophet only when he [is] acting as such.

Elder D. Todd Christofferson 
http://www.lds.org/general-conference/2012/04/the-doctrine-of-christ?lang=eng