the article doesn't go on to explain more on how this could have happened, I would assume they run some tests on the specs, or some kind of simulations (I am obviously not an expert in making submarines) but considering they are so expensive, wow, amazing. Someone put a decimal in the wrong place and nobody noticed , what are the odds of that happening!
Even in the U.S. we used to routinely have to put in slop factors to account for contractors that would routinely deliver gear in excess of spec. It wasn't until the most recent SSN class that they were able to finally hold the line against the contractors and sub-contractors... but at least the design itself would have weighed what was expected.
Say they have a CAD model with different types of steel/composites for material. If they select the wrong type of material, or the wrong weight for that material is used, or the thickness of a material is off by even a slight fraction, then an issue such as this could come up. Considering that there are millions of custom parts to a submarine, it's an amazing feat to ONLY be off by 70 tons.
