There is currently an issue with Amazon's Authorcentral. Sales figures are being misreported. I discovered this myself when a sale I know took place on a certain day was not, and has not, been reflected by their accounting software. I have informed them of the error, but it will take time to fix I am sure.
If you are an Amazon Authorcentral user for your books, please check yourself and do some verification of the data. I believe this to be more than just an isolated issue with me.