That's what I've meant: The first OS is - simplifying - the boot code. The second is the "user visible" on to which most parts of an update apply. And once updated, the update becomes part of the internal ROM and will
not be downgraded on a "factory reset". The advantage is that you don't need a so to say extra "recovery partition", what frees up some hundred MB of user space (especially with legacy 2 GB Kindles).
The other thing a reset does, is completely clearing any user configurations (most of them reside in the system area, e.g. Wifi access config etc.).
Gesendet von meinem Google Pixel mit Yabba-Dabba-Talk.
