Poor design choices?
Seriously though, I'm guessing it has something to do with Inventory. You can have an inventory-impacting invoice if it's standalone and the "logic" is probably that we'd want to show the -1 for outbound inventory.
If that's the reasoning, I don't know why that wasn't done consistently. tbh the Saved Search nodes and SuiteAnalytics ones are often not that close, so I probably wouldn't spend time trying to compare them. You'll just have to keep in mind to check on that kind of thing.