tedster - 3:56 pm on Aug 16, 2011 (gmt 0)
There is no one "right" ranking these days - as you've discovered. Logged out, logged in, different browsers, different cities, different times of day... the list goes on and on.
If you have a Webmaster Tools account, check the rankings report in there by clicking on one of the keywords. You'll get a picture of all the different ways that keyword has ranked in the past month.
Ranking reports are more of a diagnostic help these days than a core metric. Search traffic is more useful for me, and conversions from search traffic even more so.