What causes the discrepancy between the date of the shortest total daylight and the date of the earliest sunset?

Answer

The difference between solar time and mean time (Equation of Time).

This slight mismatch arises because solar time, based on the actual varying position of the Sun due to the elliptical orbit and axial tilt, does not align perfectly with mean time, which is what clocks use.

What causes the discrepancy between the date of the shortest total daylight and the date of the earliest sunset?
lightseasonYeardaySolstice