AU is an average distance from Earth to the Sun. Since Earth's orbit is elliptical, it will be closer than 1AU at certain times of year, and further than 1AU at other times in the year.
”Originally conceived as the average of Earth's aphelion and perihelion”
https://www.britannica.com/science/astronomical-unit:
”a unit of length effectively equal to the average, or mean, distance between Earth and the Sun”
I wondered whether the two are equal. A bit to my surprise, they are (http://www.farmingdale.edu/faculty/sheldon-gordon/RecentArti...)
(If we take into account that earth moves faster near its perihelion, I think that will break down)
And, nitpick, nowadays, the AU apparently is exactly 149,597,870,700 meters.
AU is an average distance from Earth to the Sun. Since Earth's orbit is elliptical, it will be closer than 1AU at certain times of year, and further than 1AU at other times in the year.