Yesterday, my reputation was 1207 and Stack Overflow is showing 1101. Why?
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The most logical reason I can think of, is that the reputation in the Account section is cached (probably updated each day). The account section show the rep of each site. Recalculating this each moment could be performance madness.
As some have already said in comments, the sudden "loss" of reputation is almost always from deleted questions or fraud detection.
See Reputation recalculation? and a bunch of other results if you search for reputation recalculation.