That’s how units work, fitting the messy natural world into comprehensible numbers. A year is 365 days, except every four, except every 100, except every 400. A month is 30-ish days, and there are 12 of them in a year, because that roughly syncs up the orbits of the Moon and Earth. Except there used to be ten of them (“DECember”), with garbage time filling in the remainder of Earth’s transit around the Sun. A second is something related to Cesium-133, because it’s close to 1/(24x60x60) of a day, because Sumerians chose base 60.