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Note also that the displayed PR is an approximation - Your PR5 could be a low 5 (e.g., PR 4.51, assuming normal rounding), while some of your links could be high PR 4s (e.g., 4.49).
When the Google toolbar became available we came up a PR5 and have remained there ever since. It has been and still is ranked #1 in Google in its keyword. A number of sites with a higher PR6 are below in ranking. Perhaps visitor popularity and/or many visitor bookmarks contribute to the position it enjoys? Don't know. Most sales conversions, however, come from outside sources referred to the site which is why we've done nothing to develop it.