Because the Imperial Upgrade is nothing to do with the Imperial City DLC, they just happen to share a word in the name.

The Imperial Upgrade is the stuff that was originally in the Digital Collector's edition of the base game. The main benefit from it is the ability to create an Imperial character (although of course how useful that is varies over time), but you also get the white horse, a crab pet and the Ring of Mara which lets you marry another character and get an XP boost when you play with them. I don't think it's listed but it also unlocks the Imperial motif for outfits and makes it free to apply.

The Imperial City DLC gives you access to the city in the center of Cyrodiil and the 2 dungeons there. It had it's own special edition which I think came with the freaky soulshriven horse and some other stuff.

If you want the white horse unfortunately the only way to get it is to buy the Imperial Edition Upgrade. It's probably not worth it if you only want the horse, but if you think you'll use the other stuff it could be worthwhile. I got it back when I first got the game so I wouldn't have to find or buy an Imperial motif (outfits didn't exist then, but I made my crafter an Imperial so he knew the style by default) and I've used everything except the Ring of Mara, so for me it was worth it.