Helmets only give the wearer a better probability to survive a direct shot to the head. Helmet engineering has improved just like other engineering disciplines. That said even a WWII helmet gave the wearer a higher probability of surviving certain calibers and bullet types more than without one. Take the example of a high angle/glancing shot. Most of the kinetic energy does not transfer to the head or helmet in either case. The head will still absorb more of the glancing forces than a helmet, thus giving the wearer a much lower probability of surviving than if they wore a helmet.

Nowadays helmets are made to improve the absorption and deflection properties, thus giving an even higher probability of survival. That said bullets of sufficient size and shape as well as shrapnel can penetrate the helmet if precisely placed.