Correcting the Pharisees, not the Law
The Pharisees most erroneously explained the words of Moses by restricting them to external acts alone, whereas they extend to internal motions also (which Christ teaches by unfolding the true and genuine sense of the law). Christ therefore corrects various things expressed in the law, but not reduplicatively (as there expressed). Rather he corrects as they were usually explained by the scribes and Pharisees (not according to the meaning of Moses, but according to their own private opinion).
Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 2 (Phillipsburg, NJ: P&R Publishing, 1992-1997), 22.