Grant,
Two things are probably fooling you a little. First, as already explained, Hornady gas check are crimp-ons. When you get your Lee sizer, you may find it is a little rough inside and you need to use some crocus cloth to clean it up a little. I chuck them in my lathe and use a Dremel tool with a felt bob and some blue Dico stainless steel buffing compound to polish the surfaces. The die is not stainless, but that compound (Ace Hardware, Amazon) works especially well on hardened steel surfaces of all kinds.
You can buy a
die for making straight-sided gas checks on your loading press. I think the
checks sold by Sage may also be straight. I've never bought any, but you can ask.
N.O.E. is a mold maker that sells a lot of other things as well.
When you measure the inside of your resized case, I expect you used the inside jaws on a caliper. Because they have flats and an offset, I've never seen those give an accurate measurement of small holes. Results that are 0.002" too small are common when trying to do that. A far more accurate approach is to measure the case neck on the outside, measure your bullet, seat the bullet and measure the outside of the neck again. Subtract the difference in the two outside neck measurements from the bullet measurement, and that's the actual resized number. Usually, it is just 0.001-0.002" below the actual expander diameter. Springback is only about that much.
This thread really belongs in the Bullet Casting forum. I will move it for you.
Oh, and a point of philosophy: Whenever I have to ask what I am doing wrong, I just ask my wife. She has no end of information on that subject.