Why is buoyant force that acts on a 46-ton ship that floats on the surface equal to 44-ton?

What you have stated in your question is not correct.

When an object is placed in water, a part of it sinks below the surface. This displaces a volume of water that is equal to the volume of the submerged portion. The displacement of water creates a buoyant force equal to the weight of water displaced. The buoyant force acts in the upward direction on the object that has been placed in water. If the buoyant force is equal to the weight of the object it floats. It should be noted that the object does not float on the surface of the water—a part of it has to be under the surface. For an object to float on the surface, its density would have to be less than that of water.

The density of a ship with the weight of 46 ton is not likely to be less than that of water. As a result, the ship does not float on the surface of water. A portion of the ship has to be submerged in the water, and the buoyant force created due to this would have to be equal to 46 ton. Else, the ship would not be able to remain afloat in water and would sink to the bottom.

A 46-ton ship that remains afloat on water has a buoyant force acting on it equal to 46 ton. A buoyant force of just 44 ton would lead to the ship sinking in water. Also, the ship does not float on the surface of water. A large portion of the ship is submerged in the water.