Walter Hardeman explained this awhile ago... probably before your time began here, Grasshopper.
It may have something to do with the fact that, once a word has pased a certain usage threshold, it will no longer show up in the normal search. It's built into the forum software to prevent massive hits on common words, or something like that. Now, I don't think 4 (or now 5) occurances should have triggered it, but who knows. Maybe there is a list of keywords it uses, too.
This is exactly why Walter added the 'exact match' option. Heck, I think there is even a post somewhere that announces that functionality. And yes, in general it should be more restrictive, but in cases like this it... isn't.
Edit:
This is the post I was thinking of.