If I understand you correctly you are talking about searching for a book with the title "Not What I Wanted" by typing those words in the search box. As in
The behavior you describe is expected and correct. Similar things will happen if *any* word in the string is NOT, AND, or OR. That sequence of words above means
Code:
NOT
What
AND
I
AND
Wanted
Said another way, it is "(not What) and (I and Wanted)".
If you typed in "Live or Let Die" (without the quotes) what you typed is interpreted as
Code:
Live
OR
(
Let
AND
Die
)
There is no other way to interpret bare keywords other than as keywords as long as calibre supports AND, OR, and NOT operators.