Marvin talking to the mattress: “I’m roughly a billion times smarter than you. For example, pick a number. Any number.” Mattress: “Ummmm…seven?” Marvin: “Wrong. You see?”
Arthur, while aboard the Heart of Gold, says, “I’m disappointed we never figured out that Ultimate Question business.” Eddie the ship’s computer immediately after says, “Pick a number, any number!”
Given the abilities of each of those characters, it’s a plausible theory. It’s also how the author chose the Answer, so at least at a meta level it is true.