That has 3/16" bearings because it wasn't designed as a rear hub. It's a front disc hub where somebody attached a cog where the disc rotor is designed to go, and also changed out the axle to re-space the hub, so that they could use it as a rear hub.

But the image you posted shows information detailing the process of doing this. It's confusing to me that you'd be using that image to show your hub without already knowing why it is the way that it is.