Isotopic Abundance and Atomic Mass

Because the atomic masses span from $78$ to $82$, the average atomic mass must be exactly the arithmetic mean of these values, namely $80$ amu.

Because each isotope contributes equally to the total mass regardless of abundance, the resulting average atomic mass is simply the median value, which does not align with $80.2$ amu.

Since the isotope with mass $80$ constitutes the majority of the sample, its high abundance drives the weighted average atomic mass to be close to $80.2$ amu.

The weighted average must be lower than $80.2$ amu due to the contribution of the lighter isotope, Se-78, despite its lower abundance.