The OP appears to assume that airline frequent flyer numbers follow some convention like Visa or Mastercard. The first digit of the former is always a 4 and the first digit of the latter is always a 5. Other credit cards use different first digits. Those in the know don't need someone to tell them what kind of card it is - the first digit gives it away.

I doubt the same consistency holds with FF numbers.

AA has changed its number scheme throughout the years. It began in 1981 with a several digit numeral-only pattern. For a while, the format was ABC1234, and then a few years later (continuing today) it was a mixture of numbers and letters.

Idreamtofly, welcome to FlyerTalk.