The neck is pressed on a mandrell, which give the inside diameter of the neck (minus some brass elasticity).
How it work ?, you may open the die, you will find a conical action of 4 jaws, like FCD. Pushing on the lever, increase pressure applied on the side of the neck.
Even with a die perfectly adjusted and a firm action of the arm.
You may or may not see a lot of work done on the neck of the case, depending on how the neck strech after firing. Try to blacken the neck with smoke to see where the die work, caliper will tell what is done.
Generally, mandrell size is perfect for cast bullet but may need a slight grinding/polishing to fit the size of jacketed bullet. Cast bullet are, at least, .001" oversized, so jacketed wont be tighten enough. (Lee have custom mandrell.)
As stated in the instructions, the die need a strong push on the arm, with practice you will develop a good feeling and a constant pressure.
You may try to apply it twice with a rotation of the case between.
I feel that the die work better when i keep pushing for few seconds on the arm at the end of the stroke, to avoid elasticity of the brass (just my tought).
If you need some practice. Have a kinetic puller handy, size few cases, seat bullets, the lever feeling when seating will give you some feedback about neck size (that is an advantage with single station press).
I had the same feeling that you when began with these dies. (I hadn't to worry about mandrell size as i only use home cast bullets.). Nothing was wrong, only the reloader which need to be used to these dies.