Learning
Multi-location inventory is stock that's split across more than one physical place, like a warehouse and a retail store, each tracked separately by Shopify.
Plenty of Shopify stores aren't just one warehouse. A brand might have a main fulfilment centre, a retail location, and maybe a smaller overflow space, all holding stock of the same products.
Shopify tracks inventory per location, not just per product. So the same variant can show different quantities at different places, and the total you see added up is really several separate numbers combined into one.
The more locations involved, the more chances there are for stock to end up somewhere it shouldn't, like a location that's been deactivated but still has inventory sitting against it (see stranded inventory).
It also means a single "available: 12" figure can be hiding something like 12 units at a warehouse nobody ships from anymore, and zero anywhere that actually matters.
Any check that only looks at a total stock number, without looking at where it actually sits, can miss this completely. Ghost Stock's location-aware checks exist specifically because of this, a variant can look fine in total and still be practically unsellable.