Там немає "чому", фіча була від початку недокументована.

   
Boolean NOT [-]

Description   
The NOT operator (.) removes all results that are in the collection immediately following the minus ("-") operator.

Examples   
This example removes all results that are in French:
lr=-lang_fr

This example removes all results that are in either French or Italian:
lr=(-lang_fr).(-lang_it)

Note: You may have noticed that the second example above uses the Boolean AND operator to specify that results should not be written in French or Italian. Depending on the placement of parentheses in your query, you could use either the Boolean AND operator or the Boolean OR operator to express this query. This is because if a document is written in French, then it is not written in Italian. Thus, your Boolean statement must express that the document is not written in French and the document is not written in Italian.

Переходьте на -и.