Skull is pretty thick and well shaped to deflect some blows away from vital bits.
The chest is a wide, flat area with significantly less protection and full of vital organs that do not stand up well to blunt force trauma or intense shock-waves.
Granted though, using 1/4 kg of TNT or equivalent, anything point blank to the chest like that is still going to take your head off. His neck was probably giblets before the pain signals could even reach his brain.