As I understand it the plated bullets are swaged from soft lead and then electro plated with a very thin layer of copper - just a few molecules thick.

Jacketed bullets have a thin copper shell several thousandths of an inch thick which is then filled with lead (by casting I believe). So the jacketed bullets are significantly harder than the plated bullets.

So, being softer, it takes less force for the lands and grooves to deform the surface and "squeeze down" the diameter of the bullet, so there is less drag (friction) as the bullet moves down the barrel. That is why they achieve a little higher velocity with the same weight bullet and powder charge.

Cast lead bullets, being even softer, will achieve even higher velocity for the same bullet weight and powder charge.

I believe that the mid range jacketed data recommendation is to make sure that the starting loads don't exceed the bullet's speed limitations. Since there is little (or no) data specifically for plated bullets, this is in line with the general recommendation to start at the lower end of the range and work your way up.