The masses in the formulas of your question, such as $$m\left(^A_Z X\right)$$
are atomic masses of neutral atoms. Those are the masses which are actually experimentally determined and tabulated. In order to evaluate nuclear decay energies, however, we need to use nuclear masses which are approximated by subtracting the masses of the electrons attached to the neutral atom. It is the mathematics of this procedure which make the number of electron masses the same on each side for $\beta^-$ decay.
On the other hand, for positron decay ($\beta^+$), the number of electron masses don't balance. There is an excess of 2 electron masses which means that the atomic mass of the parent must exceed the atomic mass of the daughter by 1022 keV for that process to occur.
Obviously, this procedure ignores the binding energies of these electrons, but the error introduced is small because the binding energies are on the order of a few eVs to a few keVs (for more massive atoms). The mass energy of an electron is 511 keV.