There can be two reasons for this:
- The product is offered by multiple sellers
In such cases they Spy will only be able to register the sales of listings whose inventory levels are beneath 999 units.
Usually the seller in the Buy Box will have the most sales and thus the greatest likelihook of having over 999 units in stock - in these cases the Spy's numbers can therefore be a good indicator of how many sales the other seller(s) outside the Buy Box are generating.
- The inventory levels of the product was above/beneath 999 units at different times
In these cases the Spy will only be able to register sales for days in which the listing's inventory levels was beneath 999 units.