The long answer:
The slightly shorter answer:
Font height is in mm (or whatever your chosen units are) but that height is the height of the EM square in which font characters are designed - they don’t have to use the whole thing.
If you want the character height to be exact, you have to type your text, then use the sizing controls to set the actual physical height.
If the “height” value you entered was literally applied to the characters you typed, your text would change size with each new character. For example, if you typed … all by itself, should those dots be 25mm high if that’s what you said the font height should be? Probably not what you’d want.
It’s an incredibly common point of confusion in graphic design software.