Round glyphs will tend to break (overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing, otherwise curved letters such as a, c, e, m, n, o, r, s, and u will appear visually smaller than flat-topped (or bottomed) characters of equal height, due to an optical illusion.
References
^Felici, James (2011). The Complete Manual of Typography: A Guide to Setting Perfect Type, Second Edition. Adobe Press. p. 315. ISBN978-0-321-77326-5.