Think multiple editions and multiple publishers. By the time The Silmarillion came out, LOTR had been out for more than 20 years in multiple editions. The individual early editions sold less, but gradually ramped up over the years, and at least two of the biggest selling early editions in the US were not Houghton Mifflin but Ace and Ballantine who were the first to publish paperback versions in the US - Houghton Mifflins earlier versions had been only hardcover as far as I know. The 1965 Ballantine edition alone had been reprinted nearly a dozen times by 1967.
