looking at Arioch images, they seem to be almost the same thing and in the first one it's clearly a unit defending the city.
You mean this?
I strongly disagree it's the same:
1) Two different views, once 3d, the other strategic.
2) More importantly, the symbol would block the civilian unit symbol possible to appear on the tile northeast, e.g. a fishing ship. I doubt two symbols would overlap in the strategic view.
3) When was it mentioned that units could defend cities from inside? Greg made a statement that units could defend a barb camp from inside* but I never heard of your own units. The only thing I can remember was a magazine stating that "units entering cities heal the HP of the city and do not defend it literally.
4) The symbol to the left is attached to the city name bar, while it could be closer and somehow attached in the right screen , but isn't. It's completely on the other tile withou relation to the city.
So IMO on the right screen it is a troop transport (civilian unit slot), and the symbol is bigger because you control this unit in the screenshot right now.
What I believe the symbol in the 3d view is: The city itself, getting a unit symbol when enemies are within range. We knew from a magazine that cities have a ranged attack of their own, if upgraded properly. See how the symbol appears slightly larger than the unit symbols around it? It also has some kind of "frame". I believe this "frame" is what the active unit gets in 3d view, it would be similar to the bigger symbol in stratview. We need to know which unit we control, don't we?
* "Camps do not have bombardment capabilities; they are only guarded (usually) by a fortified unit or two. And yes, barbarians are bound by 1upt, but remember that "guarding" a tile in Civ 5 can mean positioning your units intelligently in and around said tile"
Also, do you see that the knight under the city has a civilian symbol (triangle)? Bet it's a great general.