I thought the same (about being just flukes from being different barrels), so we actually tried another 20 inch (older) marlin, and it’s performance mirrored the same velocity as the newer production “Remlin” with 20 inch barrel. Needless to say, we were all befuddled since all logic would say that it should not be doing that. HahaI'm surprised at the 4895 results: slower in a longer barrel. 4895 is slower than 3031, which is considered 'the' 30-30 powder.
I've seen loads with 4064, but have never tried any.
Although, unless the velocity was recorded from the same rifle barrel cut incrementally shorter, I'm not certain the results are not just different barrels.
I've shot the same 38Spl load in 2 different SW M-10 revolvers (on the same day) and gotten drastically different velocities.
Who knows?