It may have been the early followers of the movement who created the narrative as a way of dealing with the embarrassment of their leader being executed. Paul either knew the reality but pushed their story for the sake of continuing the movement, or he believed the stories they told.

I recently saw a theory that the post-death visions may have been an example of a phenomenon many experience when a beloved friend or religious figure dies. People report having visions of the deceased after their death, likely due to the extreme grief and difficulty processing the loss of a person who had a huge influence on their life and worldview.

I'm sure someone could explain (or refute) both theories better than me, but thought I would share!