Macbook retina scaled resolution quality
Best for retina = every on-screen pixel is represented by four pixels on the display (1440 x 900 displayed on 2880 x 1800 pixels)

More space = every on-screen pixel is represented by 1.8 pixels on the display. (1920 x 1200 displayed on 2880 x 1800 pixels)

In between these two settings every on-screen pixel is represented by an uneven number of pixels on the display. Because there is no such thing as half a pixel, the drawing is not exact so there will be fuzziness. But the physical pixels on the retina display are supposed to smaller than we can perceive from a normal viewing distance so most people cannot notice it.

Maybe you are sitting closer to the screen than most people do. Maybe you are just perceiving a difference that is not really there. Maybe your text zooming is doing something weird. Or maybe you just have sharper eyes than most and are more sensitive to fuzziness.