What you are reading is with respect to the newest hammer bullet design, the Absolute Hammer.
The Hammer hunter and Shock hammer are a more conventional design, and behave more like bullets of like weight
The reason the Absolute behaves differently is that the nose is a little smaller in diameter. As a result, it doesn't engrave on the rifling - only the 'pdr' drive bands do. This novel design significantly reduces engraving pressure on ignition and bearing surface as the bullet travels down the barrel. As a result, under the right conditions, higher speeds can be obtained.