The reason for the earliest sunset not occurring on the winter solstice is fascinating and complex, but in simple terms: if you define a "day" as noon to noon - i.e. the highest point of the sun one day to the highest point the next day - then the AVERAGE day lasts exactly 24 hours, but most individual days are actually a few seconds shorter or longer! (It's a sine function, for those who remember their high-school trigonometry.)
Modern technological society, and industrial society before it, required precision and simplicity (for the clocks), so a day was defined as 24 hours of exactly 60 minutes each; but in fact, the length of a noon-to-noon day changes between the equinox and the solstice, and of course changes back again between the solstice and the equinox. It is because the solar noons aren't exactly 86,400 seconds apart, but the clocks still pretend they are, that the sunset starts to get later in early December while the total amount of daylight is still shrinking.
The reason for the earliest sunset not occurring on the winter solstice is fascinating and complex, but in simple terms: if you define a "day" as noon to noon - i.e. the highest point of the sun one day to the highest point the next day - then the AVERAGE day lasts exactly 24 hours, but most individual days are actually a few seconds shorter or longer! (It's a sine function, for those who remember their high-school trigonometry.)
Modern technological society, and industrial society before it, required precision and simplicity (for the clocks), so a day was defined as 24 hours of exactly 60 minutes each; but in fact, the length of a noon-to-noon day changes between the equinox and the solstice, and of course changes back again between the solstice and the equinox. It is because the solar noons aren't exactly 86,400 seconds apart, but the clocks still pretend they are, that the sunset starts to get later in early December while the total amount of daylight is still shrinking.