A razão para isso é uma meta-razão: você tem que olhar para isso a partir da história da própria série. A coleção do guia do Ultimate Hitchiker inclui o seguinte em uma introdução da Adams:
The history of The Hitchhiker's Guide to the Galaxy is now so
complicated that every time I tell it I contradict myself, and whenever
I do get it right I'm misquoted. So the publication of this omnibus
edition seemed like a good opportunity to set the record straight--or
at least firmly crooked. Anything that is put down wrong here is, as far
as I'm concerned, wrong for good.
Ele então (humoristicamente) começa a detalhar como é que o HGG passou a existir. Ele começa a fornecer uma história de seus vários (re) contos.
This is where things start getting complicated, and this is what I
was asked, in writing this Introduction, to explain. The Guide has
appeared in so many forms—books, radio, a television series, records
and soon to be a major motion picture—each time with a different
story line that even its most acute followers have become baffled at
times.
De Até logo ... ele diz especificamente (ênfase minha):
At this point I went to America to write a film screenplay which was
completely inconsistent with most of what has gone on so far, and
since that film was then delayed in the making (a rumor currently has
it that filming will start shortly before the Last Trump), I wrote a
fourth and last book in the trilogy, So Long, and Thanks for All the Fish.
This was published in Britain and the USA in the fall of 1984 and it
effectively contradicted everything to date, up to and including itself.
Assim, o resultado é que as coisas não devem ser consistentes. E é uma tradição de franquia de facto que cada releitura e adição à história deve ser diferente e inconsistente com as anteriores de alguma forma.
Francamente, a única maneira que Eoin Colfer poderia ter estragado as coisas era se ele não contradizesse categoricamente nada.