Why can it be wrong to call something nafl merely because it is not fard?

Prescribed Acts and Prohibitions: Fard, Wajib, Sunnah, Nafl, Haram, Makruh, Azimah and Rukhsah

• Because every non-fard act is haram
• Because wajib is distinct from fard yet still binding
• Because every sunnah is a conclusive fard
• Because nafl lexically means prohibition