1) Joseph Smith was not particularly good at ‘writing things down’ – when the church was organized he was commanded to have a record kept, and more often than not others kept the records, not himself. His journals are kept rather sporadically, not consistently.

2) While the First Vision is standard these days in missionary discussions, and introductions to the church, it was not always this way. The Book of Mormon was the material put forth to identify Joseph as a prophet, not the First Vision. We emphasize the First Vision for at least a few points of doctrine – the correct nature of God, the reality of revelation, the calling of Joseph as a prophet, the fact of the great apostasy – it wasn’t given this same emphasis in the early days of the church.

3) R. Lyman Bushman in his book “Joseph Smith and the Beginning of Mormonism” supposes that Joseph viewed the First Vision as an experience regarding his personal conversion. It wasn’t until some time later that he “realized” you might say, that it was his “first” vision, rather than his only one. In other words, the place of the first vision in the restoration of the gospel is something that was not apparent to him at the time, and possibly not for many years, or perhaps it’s better to say that it’s place in our understanding of the restoration was not his understanding at the time. We see it as a glorious teaching event, a stepping stone or building block – he saw it primarily as an event where he received forgiveness of his sins, and an answer to his prayer about which church to associate with.

I think it would be odd if every time Joseph Smith told it was exactly the same. That would seem like he was just rehearsing a script and not just talking about some past experience.