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Is it calculated on the fly every time a reputation score needs to be displayed, or is every user's reputation stored as an integer in the database and updated as necessary?
I believe that a user has a Rep Score that is updated any time something happens which changes rep.
Your rep can be totally recalculated using a mechanism that moderators have. This will go through everything available and give you a brand spanking new (and as accurate as possible) rep score.
I believe a running total is stored. The administrators can force a rep-recalc (which wouldn't need to be done if it was always calculated on the fly)