I do find it remarkable that with so many of us the same function produces different results - especially when Amazon are pretty firm in doing "one for all" upgrades and OS synchs.
I think it may be that we AND Amazon use words quite loosely and that Reset on one device is not the same on another, is on a different branch on the OS maintenance menu and responds differently depending upon where one launches the action.
I note that eReaders retain data pretty much regardless of what you do apart from a On Device Restore Factory Defaults. (even deregistering and then registering to a new account can leave books on there for some time until some random command seems to happen.)
On Fires data seems to disappear over a period of minutes once a device is deregistered - but that only seems to happen if it can still access (secretly) the interwebfacenet. Indeed, the only datum that seems to be retained is/are interfaceweb details.
For the record, my ancient Keyboard Kindle (2011?) just seems to keep on going whatever you throw at it. I so love it :) CW