[lathspell]

Greetings and good question, Beleglas.

I think in this case the answer is not that difficult.
Tolkien indeed was a perfectionist and kept changing his ideas and stories about Arda until his death. LotR and the Hobbit were published before he died and his son, Christopher Tolkien, published the Silmarillion later in it's latest and most compleet form. The Letters were published even later and were ideas and comments that Tolkien had about Arda, but never could work out because he died.
So, in many ways the Letters are different than the other (earlier published) books, because the ideas in them were newer or described in another way.
Maybe, if lived for another century, he might think his work perfect [img]smilies/wink.gif[/img]

hope this helps,
greetings,
lathspell