Why does phone reset to Gingerbread instead of the version of Android that came with the OS when it was purchased?
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.