Adobe Illustrator's Type Tool - Incorrect text height

Ok, so I've been using Illustrator for years, and this is something I haven't gotten around to figure out..

In my line of work, I need to be precise with my lettering. If I want a text height of 20mm, that's what I expect to get when I type 20mm in the Text Size box.

But when I outline the text to check, it's only 15mm tall.