Why should ṭāʾ and ẓāʾ not be assigned the same described contact?

Makhārij al-Ḥurūf — The Fifteen Articulation Points

• Ẓ comes from lips
• Ṭ relates to incisor roots, ẓ to their edges
• Ṭ comes from the nasal passage
• Both are cavity madd letters