The mean line (or midline) is half the distance from the baseline to the cap height. This may or may not be the x-height, depending on the design of the lower case letters. A very high or very low x-height may mean that the midline is above or below the x-height.
This confusion has been perpetuated by books (and web sites) on type which copied from books on type back to the original book or source on type that first failed to clarify this difference.
Round glyphs will break (overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing; a rounded shape will appear visually smaller than flat topped (or bottomed) shapes of equal height, due to an optical illusion.