There is an episode that explains that fact. But this is a SW site, not ST so I won't bore you all with the details.

BTW HHG is -not- silly. Maybe you were talking about the TV series, which I agree was rather silly in places, but the book is pure brilliance. The way Douglas Adams could come up with a theory to prove that a spaceship could be powered by an italian resteraunt (if I recall correctly) that was thouroughly absurd, completely logical and utterly hilarious all at the same time... such a shame that he went before his time.