Fear not, I have the explanation. I originally thought this issue was a Firefox vs IE problem (getting different results for the same search), however further research indicated that it simply this:
Google has many clusters of databases/indexes. The different indexes aren’t always perfectly in sync. So one search may use the latest database in San Francisco, and the next search your result is returned from L.A. So San Fran and LA’s don’t necessarily have in sync indexes.
See this pic where I searched for my name twice in two different windows: