The Organic Sales metric is a simple estimate calculated by subtracting advertising sales from total revenue. Though insightful, this estimate can be misleading due to the differing attribution windows used by advertising sales and total revenue reports.
Total revenue data uses a 1-day attribution window while advertising sales uses a 30-day attribution window. What this means is that advertising sales for a given day may have occurred up to 30 days in the future while total sales for a given day actually happened on that day.
As such, it is possible that estimated organic sales for a time period are zero (or even negative) due to the subtraction of future advertising sales that are not accounted for on a given day's total revenue data.
Building out a complex model to better account for these differing attribution windows is on the Perpetua roadmap and will make organic sales estimates more reliable in the future.