Check this:
I did a search on google for my top keyword and was going through each page of the search results just to see whats going on in the organic results. I saw my ads re-appear on page 5, again on page 21 and then again on page 39. When I tried the experiment over again I saw my ad again, but on different pages. (ie, not necessarily the 5th). So it looks like some of these inactive keywords ARE showing the ads but just sporadically and much deeper in the results pages.
I figure they are still experimenting and maybe are still interested in knowing how these inactive keywords/ads are performing. Perhaps they show the ads deeper into the search results because by that time the user hasn't clicked on any other ad so they might as well try mine.