Should Factory Reset return the phone to the original version Android installed on it? I tried factory resetting a Samsung Galaxy S6.  To my surprise, it left Android 7.0 on it, and didn't reset it to Android 5, which is what was originally installed from the factory.

Is this normal?  Is there a way to make factory reset force the phone to return to the original factory ROM?

UPDATE

For those curious as to why I would want to return to the original factory ROM, it's because when I take photos (even after a Factory Reset), I sometimes get a "Server Error".  I'm thinking that perhaps a bad ROM update is causing this issue, as I've already had the camera replaced by Samsung.

So I was interested in returning to the original factory ROM, and then upgrading it again to the latest version.
Factory reset only delete user data (userdata partition) and restore settings to the  default values, meaning every settings will be restored to the factory state. This doesn't apply to the system, (firmware/ROM)  which remains untouched. 

By original factory ROM, do you mean the one the phone came with? This can be done by downgrading the phone firmware: flashing lower version firmware (with a lower android version). 

Not all device support downgrading though,  since earlier versions can have bugs, security holes, etc... Many device makers such as HTC, Huawei, Motorola (Lenovo)... don't allow downgrading.