Polishing is actually a procedure introducing millions of tiny sharp edged scratches into a surface. It is the acute angle at the base of these scratches that acts as a stress concentration and crack propogation point. We are talking on a micro scale here so it is hard to imagine. Polishing acts to dazzle by light reflection but view a polished surface under an electron microscope and you will always see scratches.
Shot peening introduces no such acute angled scratches and so there is less chance of crack propogation.
There's also the effect of putting the surface layer in compression:http://en.wikipedia.org/wiki/Shot_peening
Peening a surface spreads it plastically, causing changes in the
mechanical properties of the surface. Shot peening is often called
for in aircraft repairs to relieve tensile stresses built up in the grinding
process and replace them with beneficial compressive stresses.
Plastic deformation induces a residual compressive stress in a peened
surface, along with tensile stress in the interior. Surface compressive
stresses confer resistance to metal fatigue and to some forms of
Mavic now peen some of their rims to reduce the risk of cracking.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.