Edit: I remembered that it was Arthur and not a caveman picking the scrabble pieces:
So if I recall correctly, Arthur and Ford are stranded on prehistoric earth with a bunch of original cavemen and the remnants of the Golgafrincham species, indeed the surviviors of a crashed fleet comprising of the entire useless third of the Golgafrincham species (middle managers, telephone sanitizers, etc). It appears that the Golgafrincham are displacing the cavemen who are dying out - which would ruin the computation of the question but Marvin did see a question in Arthur's brainwave pattern.
They realize that the question might still be stored in Arthur's mind and introducing a random element (picking scrabble pieces) might reveal it to them, even if the question is somewhat corrupted by the appearance of the Golgafrincham. This reveals the '6 x 9' question.
Ok, so the 'In-Universe' answer is likely to be either:
- The appearance of the Golgafrincham on the scene corrupted the computation or
- This is all an elaborate joke of Deep Thought or
- Using scrabble pieces is not the right way to get at the question
You say that you cannot spell this question using scrabble pieces, but you must remember that Arthur and Ford were not carrying a scrabble set, so they had to make one themselves in prehistoric earth, and perhaps Arthur can't remember how many different letters a real set has.
Out-of-universe: Well I suspect that just as he grabbed at a suitable answer number, he probably wanted something that was idiotic, useless and just a little bit wrong.