factory reset... doesn't
Having probably been bit by the Blueborne security bug, I'm trying to reset my phone to a known/trusted state. Ubuntu Recovery, factory wipe, then reflash boot/recovery/system.
User home directory is definitely reset. But mods I made to my root filesystem are still there! Like I added packages to /usr/lib/python3/dist-packages.
Can somebody give me an idea of where this state is kept, and why a full wipe and reflash doesn't in fact get me back to a day 0 filesystem?