How did the Kobe Steel faking happen?

Doubtless you've heard about this, but in case you haven't, via the New York Times (article by Jonathan Soble and Neal E. Boudette) 

TOKYO — Big manufacturers of cars, aircraft and bullet trains have long relied on Kobe Steel to provide raw materials for their products, making the steel maker a crucial, if largely invisible, pillar of the Japanese economy. 

Now, Kobe Steel has acknowledged falsifying data about the quality of aluminum and copper it sold, setting off a scandal that is reverberating through the global supply chain and casting a new shadow over the country’s reputation for precision manufacturing... 

...Kobe Steel said on Sunday that employees at four of its factories had altered inspection certificates on aluminum and copper products from September 2016 to August this year. The changes, it said, made it look as if the products met manufacturing specifications required by customers — including for vital qualities like tensile strength, a measure of material’s ability to withstand a load without breaking when being stretched — when they did not. 

On Wednesday, the company said it was investigating possible data falsification involving another product, powdered steel, which is used mostly to make gears. The company said the powdered steel it was examining had been sold to one customer it did not name...

So here's what I want to know - how the heck was this not caught by the customers? Is metallurgy different than chemistry? Did customers only rely on Kobe Steel's testing? (Do they not have their own QC labs?) Man, that's remarkable if so.

(Of course, how often did you QC stuff from Aldrich in grad school? Rarely, if ever, for me.)