Poking around the site I found this link from the accepted answer to this post : The global reputation recalc of March 2010.
After looking at the pretty numbers and feeling curious, I pressed the Trigger Reputation Recalc button :
I lost 301 reputation as shown in the diagrams below -
http://dl.dropbox.com/u/17036050/scr184.png
I remembered that a post of mine had been deleted and that I once heard (from the highest authority) that a sharp decrease in reputation can be caused by post deletion.
Would the rep-recalc algorithms only catch this deletion 4 days later and only after I initiated it?
Update
On the brighter side of life - a re-calc on Stack Overflow gave me an extra 8rep :P There is justice in the world ;)