Originally Posted by terrymorse

Because statistics, that's how.

Anthropometric studies of the human body have been done many times. For a normal person at a specific height, leg length, arm length, hand size, and torso length all fall within a narrow range. Certainly a narrow enough range to fit on a bicycle specified by rider height.

Also, I've personally gathered bike fit information and fitted thousands of people on bicycles. You're not special or unusual (at least your physical dimensions aren't).