I’ve always wondered about the correct usage of this word, and no wonder:
§ 90. deceptively
Would you dive into a pool that is deceptively shallow? The question gives one pause. When deceptively is used to modify an adjective, the meaning is often unclear. Is the pool shallower or deeper than it appears to be? We asked the Usage Panel to decide. Fifty percent thought the pool is shallower than it appears. Thirty-two percent thought the pool deeper than it appears. And 18 percent said it was impossible to decide. Thus a warning notice worded in such a way would be misinterpreted by many of the people who read it, and others would be uncertain as to which sense was intended. When using deceptively with an adjective, be sure the context leaves no room for doubt. An easy way to remedy the situation is to rewrite the sentence without deceptively: The pool is shallower than it looks or The pool is shallow, despite its appearance.
Per The American Heritage Book of English Usage. My Fowler’s (second edition) is silent on the matter.