There have been many complaints in the Google AdSense Help forums from publishers complaining their daily earnings are fluctuating and less than they should be.
The answer to why people are seeing huge differences in the past week or so is because Google AdSense changed how they manage the reporting pipeline.
Google AdSense engineer, Guillaume, said, "since May 14, earnings for the current day reported in the interface may now fluctuate, due to a change in the reporting pipeline."
Google has a document explaining the issues, saying that now they show gross click counts before they may discount those clicks due to invalid clicks. The document explains it well:
You might see fluctuations in earnings reflected in your Performance reports due to the lag between gross click counts and invalid click detection. Total clicks are reflected quickly and then adjusted downward for any invalid clicks, which take longer to process.
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.
This behavior is noticeable mostly for accounts with low traffic volume or a high proportion of invalid clicks.
Please be assured that after these initial fluctuations, earnings eventually stabilize, and earnings are computed in the same manner as they were prior to this change, which was effective beginning May 14, 2011. Note that finalized earnings, which are reported at the end of every month, might also reflect further adjustments.
Forum discussion at Google AdSense Help.