There is no real discrepancy here, the numbers fall within a % or two of the stated RPM. It could just be I didn't get the exact point stated where the numbers came from in the RPM range as I glanced at the graph and said it appeared to be close enough to 4500 to call it such. If you do the reverse math with the HP and tq numbers is comes out to 4612.5305 RPM's
Rick