What are your awstats looking like? Are you still getting visitors via Google? What keywords/phrases are they using?
I wouldn't worry unless it was affecting the number of visitors you are getting. Google constantly changes what it is doing, and there is no point worrying about it too much. I've noticed odd things in the rankings this past few days, so they are probably having an upheaval again.
This is just one word - it may be you rank highly for others, or for combinations. Has your page ranking changed?
I used to get quite hung up about positioning of what I thought were my key words, but then I noticed that people used an amazing variety of searches to get to my sites, the traffic keeps going up, so gave up being concerned.