The three main terrain sets: Grass, Snow; Sand (Tropic in the x-pack (as in all x-packs)) should be different and not represent the geographical location of these three civs.
Otherwise you would have: Grass, Grass and more grass.
This game is very intuitive. That means that it is made to fullfill what the most people think about the terrain, not what it in the reality is.
When you hear Egypt you imagine sand, desert and pyramids. You also need sand in every game to make it nice.
When you hear Greece you imagine europe-like terrain, that means grass and forest. Again, it's a different terrain for a different civ.
When you hear Norse you imagine snow of course. And snow is also one of the basic terrain sets in every game.
So you can associate the three civs with their terrain sets in the campaign. Or would you like to see Eggies on grass?
I think that would hurt the image about Egypt of the most people.
This also explains why the Atlanteans look very aquatic. Because it fits to the tropical terrain.
I don't see any problem in this and I would never say this should be changed.
Don't know about the mythology thing you have mentioned above, because I haven't read the help. And I probably never will *shames*, unless I have much time.