g1smd - 4:21 pm on Jul 3, 2011 (gmt 0)

I have a suspicion this happens when Google has a copy of your content, which they can show you when you ask for their cache, but surprisingly that content hasn't actually been indexed. In effect, Google doesn't actually yet know what words are on the page they are showing you. There was also some behaviour like this wrapped up in Supplemental Results handling several years ago.