Gygax answered that questions many times. Here is the first one Google found for me.

At first blush I decided that 18 was the maximum for a human, but then to make fighters more viable, and because the concpt of degrees of strength in the 18 cap followed logically, I used the percentile measurement. As for strength over 18, any such ability is superhuman and must be magically endowed in my view. The 18/% did give the fighter a real boost.

 

To the best of my recollection, I have never suggested percentile breakdown for stats other than strength.

Although, when Gary published the Cavalier class in Dragon #72, he did give them percentile values for dexterity and constitution as well. Although this was more about tracking improvements to the scores rather than to provide extended modifiers between 18 and 19 like exceptional strength.

Gygax answered that questions many times. Here is the first one Google found for me.

At first blush I decided that 18 was the maximum for a human, but then to make fighters more viable, and because the concpt of degrees of strength in the 18 cap followed logically, I used the percentile measurement. As for strength over 18, any such ability is superhuman and must be magically endowed in my view. The 18/% did give the fighter a real boost.

To the best of my recollection, I have never suggested percentile breakdown for stats other than strength.

Although, when Gary published the Cavalier class in Dragon #72, he did give them percentile values for dexterity and constitution as well. Although this was more about tracking improvements to the scores rather than to provide extended modifiers between 18 and 19 like exceptional strength.