Stain-less, not stain proof. People have issues understanding that ...

This is surprised Pikachu.

Now I know how a spot of rust looks like under an electron microscope and that stainless steel is an alloy of iron and other elements – but did I miss it, or did the article not explain why rust can still form on it, other than "alloys are never perfectly blended"?

Is it clusters of (mostly) elemental iron that are too far away from the effects of the nearest chrome atoms to benefit from the oxidization protection?