Charlie,
The search box in the forum is powered by the forums search engine, and it does not index words smaller then 4 letter words. The software can't handle indexing all of the "I, me, A, The, to, too, and, in, by, it, not, all, of, etc, can" type of short words. It's not something I can even change if I wanted to.
Tom probably used the Google search box on the home page, that is powered by Google, and indexes everything.