The reason as for why there are decimal values for atomic mass values, is because, it is the average of all of the possible atomic masses that can be formed and or present, by taking varying number of neutrons. This varying number of neutrons causes different mass values to be present and by taking the average of the atomic masses, from all possible isotopes, the value yields an average or approximate atomic mass based on all of the atomic masses formed from their unique number of neutrons.
Electrons are not included as they are not found within the nucleus of the atom, for which the atomic mass value can be obtained. They are found outside the Nucleus, within orbitals, of regions of space.
