Why are there Differences Between Joseph Smith's 4 First Vision Accounts?

I have just been made aware of a very well-done post on LDS.net on the differences and similarities of Joseph Smith's 1832, 1835, 1838, and 1842 First Vision Accounts. One can find it at this page. It also features links to these different accounts of the First Vision, so one can read them for themselves, which I highly recommend.