Type on a screen is read in lines, and the length of a line decides how hard the eye has to work. A line that is too long leaves the reader hunting for the start of the next one; a line that is too short breaks the sentence into so many pieces that the eye spends its time on returns instead of words. Somewhere in between, reading feels effortless, and nobody notices the typography at all.
The unit ch is the width of the digit zero in the current font, so a measure of 65ch is about sixty-five characters, not exactly. Letters vary in width, spaces are narrow, and every typeface has its own proportions. The numbers above are counted from where the browser actually placed each character.