AFAIK, it's from different items coming from different Amazon warehouses, and nothing to do with "overstuffing".
In your case, one ring pack came from the same warehouse as the wolves, and was the last one. So the other one had to come from a different warehouse and so was packaged on its own.
Amazon tries to distribute its stock across all its warehouses so there's hopefully an item nearby for when you want one-day delivery. But with products that aren't super popular, there's often just one.
Note that if you delay shipping with Amazon Day or no-rush when available, your items are more likely to come in a single package, because that gives Amazon the time to send items from different warehouses to a single one and then package all in one place. This seems to be based on a complicated calculation of whether that will save Amazon money, so it doesn't always happen.
In your case, one ring pack came from the same warehouse as the wolves, and was the last one. So the other one had to come from a different warehouse and so was packaged on its own.
Amazon tries to distribute its stock across all its warehouses so there's hopefully an item nearby for when you want one-day delivery. But with products that aren't super popular, there's often just one.
Note that if you delay shipping with Amazon Day or no-rush when available, your items are more likely to come in a single package, because that gives Amazon the time to send items from different warehouses to a single one and then package all in one place. This seems to be based on a complicated calculation of whether that will save Amazon money, so it doesn't always happen.