When you subtract the atomic mass from the atomic number what do you get?
When you perform operations with chemical properties, really consider the meaning of what you are doing. What does it PHYSICALLY mean to subtract the atomic mass FROM the atomic number?
The atomic mass is the average mass of all known isotopes of an atom based on their relative abundances. The atomic number is only the number of protons and is NOT an average.
It gives no physical significance to subtract an average mass from the number of protons.