Thanks for visiting us here in the Intuit Community, Quickerbook,
Let me get the help you need about Assembly and Item Quantity.
Just to verify, did you mean the build assembly item shows quantity available than your item inventory quantity?
If yes, please know that in QuickBooks Desktop, when an assembly build is performed, the components are deducted from inventory and the quantity of assembly items increases accordingly.
Therefore, after creating an assembly, it will increase the Quantity of the assembly and decrease the Quantity of the inventory parts used to build the assembly.
Here's a helpful article for more information: Create, build, and work with inventory assembly items.
Please don't hesitate to reach back out if you have further questions concerning Assembly and Item Quantity, I'd be glad to further assist. Have a great day!