Why does phone reset to Gingerbread instead of the version of Android that came with the OS when it was purchased? When I first purchased an Android phone over a year ago the installed OS was Android 2.1 (Eclair). The OS has been updated twice by the carrier since then and now runs Gingerbread.  However, I had to restore my phone to factory settings recently and the OS that was reinstalled was Gingerbread. I was expecting it to restore back to 2.1 (Eclair).  Is this expected behavior and was the phone reset properly to factory settings?  I followed the hard reset instructions here and everything looked correct. However, this was not the factory restore I expected.  If this was correct does anyone know how the Gingerbread OS "replaced" the OS that was installed from the factory?  I thought the factory settings were "read only."  Thank you very much.

This is all from a stock Android phone that has never been rooted.
Resetting to factory settings removes all personal settings, data and installed apps from the phone.

It doesn't uninstall operating system updates and doesn't remove apps that your manufacturer or service provider have included in the system.

It also (normally) doesn't delete data from your SD card.

Installing operating system updates on an Android phone overwrites the previous version of the Android operating system and doesn't keep any of the old OS files. It's the same as if you upgraded a Windows Vista PC to Windows 7, you can't go back to Vista on that PC without finding the original install disks, as it has overwritten and deleted the old Vista files. The only way to go back to the previous version is if your manufacturer/network has made a copy of the old OS files public that you can download and install.