to accurately meausre crank hp you'd need an engine dyno, not a common dyno that you've used. Crank hp is always calculated and is just connected to too many variables/settings on normal dynos to be reliable. Hence all that should be quoted is whp, and again hope that the dyno settings are correct in the first place.
its calculated by measuring transmission losses on the dyno, its not as if they just apply some magical formula to it. If other cars using that same dyno get to their given hp (e.g. the TDI engines from the VW group) and others dont (e.g. the Z, or the RX-8 which apparently is a complete disaster on that level, or the S2000) there are some conclusions to be drawn
But i agree, the most accurate method is by using an engine dyno.