No one said overvoltage is a failure mode. You took a list of things that can go wrong, for some reason created a cause-and effect between two random things on the list, just so you could correct someone?
a 9V battery should produce around 9V. There are plenty of recheargable 9V batteries sold that are over 9V, because 9V is either using lithium or nicad in series - none of which add up to 9V. The 9V lithium batteries I buy have a tiny voltage converter and deliver 9V. If I order that same battery from a random ebay source, they can easily save on the voltage converter and deliver 10.5V for example.
More to the point of my example, a bunch come off the production line with a Broken voltage converter. They don't pass QA and get sold on ebay.
a 9V battery should produce around 9V. There are plenty of recheargable 9V batteries sold that are over 9V, because 9V is either using lithium or nicad in series - none of which add up to 9V. The 9V lithium batteries I buy have a tiny voltage converter and deliver 9V. If I order that same battery from a random ebay source, they can easily save on the voltage converter and deliver 10.5V for example.
More to the point of my example, a bunch come off the production line with a Broken voltage converter. They don't pass QA and get sold on ebay.