PPC sales can, in theory, often be attributed to two different days:
1. the day when the click on the (sponsored) product occured, and
2. the day when the product was actually bought, after it was initially saved to the shopping cart, for instance.
The tool will show PPC sales on the day of the click, as well as organic sales on the day of the actual sales. As this would be calculated twofold, the tool has to substract the organic sales from the previous period (when the clicks happended) - this is also the way it is handled by Amazon.
This way it can be guaranteed that the overall sales summary remains intact.