Pretty simple really. Nords, Dunmer and Argonians allied. I'm not trolling, but I may be an ass. If you know anything about the lore, you know this is absurd. They will invent some reason as to why it works though.
it does work, There have been border disputes depending on who was ruling what and at what time, argonian occupation, the battle of red mountain, but things change. unlikely allies are formed all the time, its just strategy.
sometimes you need to make peace if you want to achieve a mutual goal and benefit from the strength of your rivals.
Also the point that was brought up about Cyrodiil looking the same is valid. If you knew the lore, you'd know that during the time that ES:O will be taking place, Cyrodiil was supposed to be a jungle. You can't apply Earth's geography to Nirn. Just because our geographical features don't change much over time doesn't have any bearing on what happens on a magic filled world like Nirn.
you have to be an idiot to think that BGS would not be providing lore support and artwork to work from.