The main reason is due to the sales attribution window (Seller Central is 7 days and Vendor Central is 14 days). This means that someone can click on an ad on day 1, and anytime in the next 7 days (14 for vendor) if a user then buys a product, the sale is attributed to that ad. This means that if the end date of our analysis is, for example, January 31st, the ad sales for the period January 25th - 31st inclusive (January 17th - 31st for Vendor) is subject to change if a user converts after clicking on one of your ads. For more information, please see here.
Another reason may be due to the click attribution window of 72 hours. This simply means that click attribution to a sponsored products ad may be given or taken away up to 72 hours after a click occurs.
However in our experience, both of these adjustments are not large and are unlikely to change the overall analysis in the audit.