Text is measured from the Em-square. So if you have a text with large acenders and decenders the size of the letters may differ from the size of the text. The Em square is the block of metal that the type would have been cut out of,
(see https://community.adobe.com/t5/indesign/how-to-determine-a-text-size-by-actual-print-measurement/m-p/11227043)