To Slamfire's point on rate of increase in velocity per unit charge weight as the load gets warmer, the fact some powder measures don't handle flake powders as precisely as some others, with errors of a third grain being common and half a grain not unheard of, it may be that one reason for the 5.0 grain limit by one load manual while another goes higher has to do with leaving some headroom for charge dispensing errors.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Benefactor Member