Let me take a stab at pshat here. When we talk of a chatzita in hair, it can be that it is tied up in knots. When we talk about a chatzitza in the body, it can be like mud.
Let us argue that the waist is the midpoint.
Suppose that up until just below the waist, a person is covered in mud. Suppose that above the waist, a miyut of hair is knotted up. However, when you add the amount of (square inches of) the person covered with mud, to the amount that has knotted hair, the person now has a chatzitza on rov of his body. The Rambam may say this is a chatzitza, while the geonim would say since each type separately has only a miyut, it is not a chatzitza.