New optimization technique: Change your title every time Googlebot crawls so that you rank for all the terms. ;-)
We've certainly seen Google present different titles for different searches. I've certainly see Google generate titles with the keyword. I've seen them add brand name to a title. I've seen them shorten the title. This is the first time I've considered old titles as a source.
Is it showing your title or their title? That is, does it pull up a cached Preview with "green widget" or does it just say that in the SERP? There are threads going back at least a year about how the "title" shown in search results isn't necessarily the actual title of the page.
Beyond all the discussion we've had over the past years about how Google has been changing titles, the original post is very lax about relevant details, so we really don't know what particular factors might have been involved. Eg...
- How long prior to the observation had the title change been made?
- Did any prominent content in your page (like a heading) reflect the original "green widget" title?
- Does your less prominent page content still reflect the original "green widget" title?
- Are there external inbound links or internal nav links to the page referring to green widgets?
See this discussion for some recent background about title rewrites...