Think of an eye chart twenty feet away, and a person reading the small line without trouble. That is the picture the two numbers carry: sight that is completely clear. English then takes that clearness and points it in a second direction, backwards in time.
可以想成二十英尺外的一張視力表，有個人輕鬆讀出小字那一行。這兩個數字帶著的就是這個畫面——看得完全清楚。英文後來把這個「清楚」轉了一個方向：不是往前看得清楚，而是往回看得清楚。
From the word’s history
From a method of testing vision by reading charts at a distance of 20 feet.
這個說法來自視力檢查的方法：站在二十英尺外讀視力表。讀得和一般眼睛一樣清楚，就記成 20/20，所以兩個二十一個是距離，一個是比較的標準。
Wiktionary, "twenty-twenty"