Not at all my field, but I would expect that a ship with a "50 ft. draft" means that it draws 50' when fully loaded and ballast tanks full. Not being fully loaded, or being in calm enough water (ie. not at sea) to use less ballast (temporarily) might allow the ship to draw less than 50'.
Am I wrong?
No offense, but yes, you're wrong (a little).
I've enjoyed this thread because it has taken some interesting turns into subjects I don't get to expound on very often. And some good questions, generally speaking.
So draft, ballast, load lines, trim and stability (where did that come from?).
The quoted draft of a ship listed in stats is typically "summer draft" or max draft allowed, as boat geek says. But what that means is that the draft marks on either side midships are as deep as you may load the ship. Every commercial vessel has them. (How they are placed is another long story, but it basically is an issue of safety, angle of roll, or incline, and required free board to avoid deck edge immersion and down flooding risks.) This allows you to determine how much weight you can load on a ship, including cargo, fuel, ballast, stores, etc. But to really get that maximum weight onboard, you have to have your ship on an even keel, fore and aft, and straight up (no list to one side). This means you have to distribute the weight so that the ship sits nice and flat. But a ship doesn't sit flat. It sags in the middle, or "hogs" with too much weight at the ends, causing the middle to arch up. So it has to be distributed just right to maximize the cargo lifting ability of the ship. That can be hard to achieve perfectly.
Loading to your marks, or maximum draft, is not usually reached. This is due to the volume of many cargos taking up all available space, before reaching the maximum weight able to be loaded. Typically, a ship has to be designed to carry heavy cargoes to reach it's marks regularly. A bulk ship that carries grain, ore, or other commodities does it often, but most other ships don't.
I don't know what the drafts of the Dali were when it left the dock, but I'm sure it was deep, but not to it's marks. Looking at pictures, you will see the red bottom sticking out of the water near the stern. That is not because it is light. That is because it has a bridge resting on it's bow. Bridges are heavy. Push the bow down, and the stern goes up. (It's like a seesaw.) My guess is that if it had a max draft of 50ft, it might have been around 45ft. While it's cargo might have been light, since we don't export as much as we import, the ship had a long voyage ahead, and probably had a large fuel load. Fuel can have a significant affect on a ship's draft. That thing probably burns about 200 tons a day at sea, and had about a 30 or 40 day trip ahead of it. A full fuel load would be significantly more than that.
Ballast - ships don't get paid to load and carry ballast. It causes a ship to have poor fuel economy since you increase the volume below water and that is harder to push through. Plus, with cargo on board, it can put you below your marks, which is not legal. A ship loads ballast to trim it in a desirable condition (flat, or slightly down by the stern). Also to provide positive stability. In other words, keep the ship from being top heavy and rolling over. Ballast can also be needed to ease stress on the ship's structure. You can easily break a ship in half if stressed too much by weights not being distributed well. Ballast levels out that distribution. But the whole "keep it from rolling over" thing is the biggest reason on a ship like the Dali. See all those containers stacked up high? That raises the center of gravity, and if it gets too high, the ship rolls over like a cheap toy in a bathtub. So they usually load some ballast down low to keep "positive stability." That cannot be removed if stability is low.
An empty ship will also load ballast to sink down lower so the rudder and propeller are submerged properly. On rare occasions, an empty ship may load additional ballast to keep it from being damaged in bad weather (bobbing like a cork is bad). But that would be temporary, and only on an empty ship.
So ballast is often necessary, but you never want to load more than you have to. As I said, a deeper ship takes more power to push through the water. It costs more fuel, which is expensive. So the ship likely didn't have too much room to remove ballast. And depending on where and how much cargo is loaded, you might not be able to load any more if you wanted to. Not without excessive stress or too much trim in the wrong way.
I hope this makes sense. I tried to keep it as simple as I could. If it's confusing, let me know how so and I'll try to do better.