It's tough to nail down the exact size of a small asteroid speeding through space. NASA's close approach database lists diameters as a probable range, such as 11 to 23 meters. We included all asteroids with possible maximum diameters greater than 20 meters but for brevity display an average. In some cases, the average is less than 20 meters.
Why do some asteroids have the exact same flyby distance?
Just as it's tough to determine the exact size of a small asteroid, it's hard to calculate the exact distance at which it passed Earth. NASA's close approach database estimates flyby distances as "nominal lunar distances" where one LD equals the average distance between the Earth and the Moon. Nominal LDs are only estimated to two decimal points, giving some asteroids identical flyby distances.