difference between mass number and atomic mass

the mass number is the number of nucleons in an atom, the sum of protons and neutrons in an atom, if an atom has 2 protons an two neutrons, the mass number is 4,
 while the atomic mass is the actual or we can say the measured mass of an atom,  it is the mass we obtain when we use a scale or instrument to measure the mass of an atom,
 the atomic mass is different from the mass number for the following reason

1.  the effect of binding energy, when calculating mass number we just count the number of nucleon but , for atomic mass part of the " sum of nucleons"  is lost as mass, this is why atomic mass and mass number are not the same.
2.  the the mass of protons and neutrons are not unity, , proton is about 1.08 and neutron is more than this, but we count them as one therefore there are excess mass will count for the in the atomic mass