The perp was carrying a kidnap victim.
I SPECULATE that she may possibly have received training which hypothetically suggested that in such a case, the defending citizen should shoot the perp in the legs to avoid injuring the kidnap victim.
I think she was taught to *say* she was aiming for the knee.
“Hit him in the chest? Darn, I was only aiming for his knee.”
That makes sense. She could not shot the center of mass without risking her own child.