Suppose I'm advertising on Adwords, and targeting the phrase "wordpress themes". I'm using the negative keyword "free" to avoid the freebie hunters as I'm selling paid premium themes.
Suppose a searcher is in the process of searching for "wordpress themes free". At the point they type "wordpress themes", my ad pops up and gets a click. My visitor actually wants free themes but never got as far as typing out "free" because they were seduced by my ad.
I imagine that often negative keywords come last in a phrase, giving the searcher ample opportunity to get sidelined before they complete their full search phrase.
More money for Google? Fewer satisfied visitors for me?