Why was the Titanic called the unsinkable?

White Star Line claimed after the ship sunk that they did not say the ship was unsinkable and that people over heard them saying it could almost be unsinkable and so it spread around like Chinese whispers and changed to the ship was unsinkable.

White star line said before the departure of the ship : 'these two wonderful vessels are designed to be unsinkable' So people got the impression that the whole ship was unsinkable.
The Titanic was called The Unsinkable Ship because she was designed to stay afloat with 4 watertight compartments breached. But when Titanic collided with the iceberg, 5 compartments were breached
The builders of the ship (Harland and Wolff) insist that Titanic was never said to be an unsinkable ship. They say that the myth was a result of some articles in a few magazines.

The Titanic had several watertight compartments, which would be sealed with automatic doors if water came inside. The Titanic could in fact stay afloat with four of the sixteen watertight compartments filled with water. The automatic doors would make sure that the water stayed in a limited area of the boat if water came in.

The Titanic sank because the iceberg created a crack along several of the watertight compartments, filling several of the compartments. In addition, the compartments wasn't as watertight as everyone thought. The water flooded over the top of the compartments, and filled even more compartments.
before its departure, everyone knew that the boat was built to go against anything, maybe the best built ship there was, that is, of course until it hit the iceberg. Some people refused to get off the ship as it was sinking because they knew that the ship was hard, and strong, and refused to believe that it was sinking. But the titanic was made of iron anything really sharp could of sunk it
Because it was made of a very strong and stable metal, so they thought that it would make it impossible to sink. However, it was made of iron, and iron can be shaped.