This is an oft repeated factoid with lots of variations (my favorite is the tale that Tolkien's editor underlined "dwarves" and corrected it to "dwarfs" citing the OED. Upon seeing this correction, Tolkien allegedly replied "I wrote the OED, and I said dwarves"). This link seems to be a good resource in explaining the reality behind the difference: he was writing a lot very quickly, and made a mistake based in familiarity of plurals rather than philology or linguistics.