It isn't really. The difference is that an ultralight is typically something that just doesn't have any lifting strength in the blank. They are usually much, much too light to manage a fish of any size other than crappies, or panfish of other sorts. A dropshot rod, by character has some lifting power in the butt, it's got some strength to move fish.
I think the lightest rod I fish with any regularity is my SMR750S. That rod is wicked powerful, more than many mediums. It's got enough power to move riverine brown fish up to about 3.5lbs with no issue. I love that rod for fishing grubs and the like on clear lakes. If it were any lighter, though, it would be a push.