When a hauler is carrying good to the storage, this amount is not reflected in the available amount of the storage. However, it will eventually show up there. The scripts that depend on the workshop input (ingredients) will benefit if the returned would be not the real amount that is currently in the storage, but the amount that will be there in a reasonable time. It's better to delay worker release / building pause until the good arrives than prematurely stop the workplace work.
Still need to think if we ever need the input signals for the real storage amount?