How can your shaving schedule be an indication of testosterone?

Very rough estimate! Many factors go into shaving schedule including genetics & testosterone. Someone can produce lots of t but genetically not have much facial or body hair. Others might have less t but genetically be prone to more hair. With that said, t is converted into Dht (dihydrotestosterone) which is androgenic - medical speak for making you more masculine. So perhaps more t leads to more Dht thus more hair.