Oil is heavy. The ship designs are known. You can tell approximately how full a ship is by how deep it sinks into the water/where the waterline is. They can pr

Oil is heavy. The ship designs are known. You can tell approximately how full a ship is by how deep it sinks into the water/where the waterline is. They can probably consistently lie by a small percentage and Iran let's them get the 3% discount as an acceptable loss.