In the manufacturing process you have several methods that are used each with their own issues. Photo-lithography suffers from diffraction at the edges of features. Chemical processes undercut, overcut, and generally move around. They eat away faster at edges because of surface space and eat along different crystal lattice vectors at different rates. Plasma processes are effectively a hose of charged particles and have a sort of Gaussian distribution vs location of particle impact depending on your focus. And during the whole process, diffusion is trying to move everything around to boot. These effects exist at all scales, but when you get to IC scales, the effects can be dominant.