Why do cylinders in IC engines get out-of-round as the engine is operated for a long period of time? Why is wear on the cylinder walls not the same along the length of the cylinder? Theoretically, why...

Why do cylinders in IC engines get out-of-round as the engine is operated for a long period of time? Why is wear on the cylinder walls not the same along the length of the cylinder? Theoretically, why is piston frictional force equal to zero at TDC and BDC? In actuality, why is piston frictional force not equal to zero at TDC and BDC?