Except for the explanations already offered, I don't know how to explain the difference between load data for jacketed vs. cast projectiles. However, there are a few assumptions I make, when comparing data for one bullet type, to arrive at data for the other:
1.) All other factors being equal, bullets cast from homogenous lead:tin:antimony alloy will tend to create less friction against the barrel than projectiles that are jacketed in gilding metal.
2.) Cast bullets that are polymer coated will tend to exhibit still less friction against the barrel than uncoated conventionally lubed cast bullets. Depending on the cartridge and pressures involved, I usually observe a 2% - 5% increase in velocity, all other load characteristics being equal.
3.) Given equal bullet weights/diameters/lengths, a given charge weight of propellant will tend to produce lowest velocities with the jacketed bullets, followed by the conventionally lubed cast bullets, followed by the PC'd bullets. Pressures tend to run in the opposite direction.
4.) Given identical bullet weights and configuration, jacketed bullets will tend to be longer than cast bullets, because the gilding metal jacket has lower density than the lead alloy. Longer bullets mean greater seating depth, which means less unoccupied space in the cartridge. This can translate to higher pressures. In larger capacity cases, this difference may turn out to be negligible. In smaller-capacity cases, a slightly greater seating depth can mean a radical increase in chamber pressure with attendant unfortunate results.