Small caps are short capital letters designed to blend with lowercase text. They’re usually slightly taller than lowercase letters. (Small caps are used in this book to denote cross references.)
But most people have never seen real small caps. They’ve only seen the ersatz small caps that word processors generate when small-cap formatting is used.
Small-cap formatting works by scaling down regular caps. But compared to the other characters in the font, the fake small caps that result are too tall, and their vertical strokes are too light. The color and height of real small caps have been calibrated to blend well with the normal uppercase and lowercase letters.