There are two primary reasons why you may see different inventory numbers in Inventory Manager than you do in Seller Central.
Helium 10 receives an updated Inventory Report from Seller Central once per day. New sales or changes to your inventory levels that occur on the same day after that report is generated aren’t reflected until the next report is received and processed by Helium 10, generally the following day.
When viewing the Current Inventory field in the Restock Suggestions page, it is important to understand that this number represents:
Amazon provides us this information, however, we group and present the data slightly differently. Inventory Manager's goal, regarding Current Inventory, is to give only the most accurate representation of how many units you have that can be sold.