I use premier so maybe something is different in enterprise, I have one column, for assemblies
that number comes from pending builds, you have told QB you will need those items in the future for a build
On sales order means you have promised that qty to a customer
So qty available is the qty remaining that is not "reserved" for a customer or build.
Actually QB does not reserve anything, even if the qty need for a build is 5, and 5 are stocked, you can sell those 5
In menu edit>preference>inventory>company tab you can select whether or not you use on sales order and pending builds in the available calculation.
Would you post a screen shot of the report showing a column for to be built, and for assemblies?
