On Adobe Support Community:
"A typeface is designed on an imaginary relative space called the em square. When you specify type size in software, you are specifying the height of the em square and the glyphs are scaled proportionally and positioned relatively to it.
The reason is not just an archaic throwback to pre-computer type casting.
Even in a single typeface, not all glyphs measure the same height. Rounded characters are taller than other characters. Ascenders and descenders differ in their heights.
Typefaces are designs. They vary widely in stylistic shape; geometric sans-serifs to flourishing scripts and everything in between. In a line of type, if you change a word or phrase to, for example, the italic version of the same font, the actual height measures often differ from the corresponding regular glyph. So you wouldn't want the specified type size to be an actual measure of the glyphs; you'd want the glyphs of the italics, punctuation marks, symbols, etc. to be correctly proportional to the non-italics.
So the em square serves as the "proportional equalizer" within a typeface. The actual glyph shapes are drawn in relative position to the em square, but are not necessarily constrained to fit within it. A flourish of a decorative script may exceed the bounds of the em square both horizontally and vertically (something not even possible in the pre-digital physical type slugs often cited as the reason for why type size is not specified according to the actual height of the glyph outlines).
The problem with using Illustrator's Use Preview Bounds setting is that it does just that: It simply causes the height and width fields to display measure values of the "painted" result of effects applied to the paths. So using it to set the height of text with the Outline Object effect applied will yield differently-sized results, depending on which characters are typed.
Usually, when someone wants to specify type by the "height of its characters", they really mean they want to specify the measure between the baseline and cap height (two guides within a typeface usually corresponding to the height of non-rounded capital glyphs). This is a common and legitimate need, for example, in the sign trade. CorelDraw provides an option to do just that, and it's one reason why Draw is popular within the sign trade."