Inventory Velocity Report

The Inventory Velocity looks interesting.

https://documentation.vin65.com/Reports/Reports-List/Inventory-Velocity

30 Days/ 60 Days/ 90 Days The number of products sold in the last 30/ 60/ 90 days.

But why would a product number sold go down form the 30 days sale to 60 days and yet go up again for the 90 days? Such as 30 days: 50 bottles, 60 days: 25 bottles, 90 days: 100 bottles.

Any ideas?

Blessings,
Betty