Friend of mine is a retired (and later re-hired by a former competitor) engineer with the big motor companies who has worked on their engines all his life.
He once told me manufacturers are allowed a 10% tolerance between the stated H.P. and the actual H.P. According to those charts, it sure seems like what he claimed is supported by the charts. Unfortunately, the tolerance seems to only be on the low side, as none of them are even close to their stated H.P.