The average mass of a nucleon is taken as one amu. Any element necessarily consists of an integral number of nucleons; still the atomic mass of an element (on amu scale) is not an integer, primarily because

of the pressure of extra nuclear electrons

of the existence of isotopes, with different relative abundances.

of the mutual conversion of proton into neutron and vice-versa

the mass of a proton is slightly less than that of a neutron