I get higher than book velocities with my 44 Mag RH also. Mine is 7.5" and the books usually use a 4" barrel for testing. Forcing cone and chamber dimensions may well help it also.
Be happy about it. You can run lower pressure ammo (usually more accurate) with maximum results. While the guys with 4" barrels are shooting their one cylinderful of Buffalo Bore ammo for the day, you'll be breaking out your 3rd box of 50 (lol) and besting them through less recoil and just as much (practical) velocity.
Time to buy a mould! RHs love lead.