It's not just AMEX. My Chase Premiere Visa recently declined a $10k charge to pay for my daughter's tuition. I had plenty of available credit to cover the charge, so I was puzzled why it was denied. I called and they said it had been tagged by the antifraud department. The CSR then asked me a series of bizarre multiple choice questions involving my personal history. I passed the test and they allowed the charge.

I thought it was very interesting that they apparently knew what address I held back in 1988 and what car I am currently driving (despite the fact that I paid cash for the car and didn't finance it), but they didn't recognize the name of my daughter's current high school.