These two forms are close because both are qualified by the subject's titular description, yet their logical force differs.
General conditional: the relation is necessary while that description holds.
General customary: the relation is permanent while that description holds.
Suppose a “teacher” is considered precisely while teaching. “While teaching, the person continually speaks” may be a claim about continuity, but it does not thereby express logical necessity; one may teach by writing or gestures.
The key is to separate the duration of a condition from the force of the predication.