The cosmic inflation did resolve the flatness problem by the theory which states that the universe appears to have a flat geometry.

The difference between roughness and flatness is that one, flatness, is the shape of an object. Roughness is the surface or texture of an object.

What causes flatness on the parietal bone at the back of the head

a measuring tape

The horizon. When staring at the sea you will see that it goes without end. The sea vanishes in the sky.