Since May 14, 2011, earnings for the current day reported in the interface may fluctuate in the Performance reports "due to the lag between gross click counts and invalid click detection" according to Google. More detail was provided in a help center article on fluctuations in estimated earnings:
After clicks occur, estimated earnings are increased based on these recorded clicks. However, processing for invalid clicks is completed after this initial estimate, and the corresponding earnings might then be revised downward in your reports.
Google indicated that finalized earnings, which are reported at the end of every month, might also reflect further adjustments.