>This was the sixth time that exact piece had broken off and hit the vehicle.

That was part of the problem and what is meant by the foam shedding being "in family." They had witnessed it enough (along with foam shedding elsewhere) without consequence that it wasn't really considered a credible risk. Until it happened during a period of the launch where the delta-v made it a different scenario, with an energy that caused unexpected characteristics about the foam.

This is second hand knowledge, but the person I know involved in the testing said they had a really hard time recreating the damage to tiles on the specs they were given after Columbia. By chance, they decided to turn the gun beyond the quoted specs (as I understand it), and all of a sudden the foam acted like a hard chunk of debris. I think it's hard for people to grasp how much is unknown, even after a disaster. Often, it's only in hindsight where it seems obvious.