I actually think the likelihood of a malfunctioning rocket being fired over Japan is extremely small. N Korea understands that these tests make them just enough of a threat to their enemies that they won't attack N Korea. But if a test goes wrong and one of those missiles drops out of the sky and lands in Japanese territory, Japan and their allies (America first in line) now have a valid reasoning for invading and dismantling their government and military. N Korea is likely only launching their absolute best rockets with the above understanding about how perfectly things have to go for them.