What Raptor calls a 12psi pulley would be 12psi on a totally stock engine. As delco already mentioned, boost is just a measure of restriction, better heads, better cam and exhaust and the engine becomes much more efficient.
Just have look at some of the L67 engines, again they have different pulley sizes, up to 19 psi, however on a well built engine, even with such as small pulley you will never actually see 19psi boost in the manifold.
The other thing to consider is, the raptor is the equivalent to about a mid sized vortech blower, you would need to find out which vortech blower they used to actually achieve 12psi on the same engine.
The other thing, as delco mentioned, cam timing.