The easiest explanation is that Tolkein had crafted a very complex work -- with a chapter on the elvish language IIRC -- that while being uber-geek, was bound to have holes.

Now, for a truly interesting question, how was it that the First Foundation was completely unable to figure out that the Second Foundation's home was on Trantor -- because no one on Terminus was able to recall the phrase, "All roads lead to Trantor, and that is where all stars end."?