It may have made a difference, but switching between standard jacketed bullets isn't usually a problem like switching between a coated bullet and a standard jacketed bullet. You won't know until you shoot the 75gr bullets from a slightly fouled barrel. I say that because most H&Rs shoot best from a fouled bore, not unlike a lot of other makes of firearms. Most likely it just doesn't shoot that load well. As it gets broke in and the bore simmers down a bit, it may shoot them both very well. Mitchell's 243 Ultra shoots 55gr and 95gr Ballistic silvertips extremely well, but it's got a lot of rounds thru it being he's the second owner, Mac owned it before and it shot extremely well for him with the Win 95gr Supreme factory ammo as well as another handload he made.
Tim