“Now Moses was faithful in all God’s house as a servant, to testify to the things that were to be spoken later, but Christ is faithful over God’s house as a son.”
Servants and sons operate on different levels, says the writer to the Hebrews. Two differences right there in this single verse. First, Moses the servant was, Christ the Son is; that is to say the Son still being faithful when the servant has long ceased from service. Second, Moses the servant was in the house, Christ the Son is over it. Those are different spheres of responsibility.