Why do chips not always "meet the grade"?
Modern ICs are REALLY small. Tolerances are huge during processes such as ion implantation and oxide growth. At such small sizes, these things can't be treated as anything but a probabilistic process. Lines also tend to be smeared due to the feature size being the minimum possible given the wavelength of light. When you get worst-case performance in a bunch of these different steps, then you get a non-functioning IC.

Companies don't design the IC so that it functions at the worst-case - it would be too costly. So instead they do Monte Carlo simulation of the manufacturing parameters, estimate a yield, and do testing after the fact.

Typical "design corners":


Source: What I remember from my IC manufacturing class.