A teacher teaches a subject , 

he is an expert and professional in his subject 

he follows the professional ethics 

a master, a guru teaches gyan , 

here our context is gyan

and concern is on the precision accuracy of the stuff delivered 

any personal imaginations surmises cannot be gyan and master must necessarily possess the authenticity of direct experience of absolute to be able to, to be eligible , to teach gyan if not followers will be grossly mislead