The taxi driver tan takes its definition from the job group it's more popular with, just like the farmer tan.
As the taxi driver usually stretches his arm and rests his elbow on the open front window in the summertime, his arm gets a much stronger tan than any other parts of his body.
Therefore, you know you're being fooled with when you get hit on by a man in a holiday resort who got one arm pitch black and the other significantly less tanned, who claims he's a billionaire with his own private yaucht.