Right. The expander exists for a standard die or for a bushing diameter that matches the neck of a standard die. If you look at SAAMI rifle drawings, they typically allow a -0.008" tolerance for case diameters, including where the neck is over top of the bullet. That means up to 0.004" of spread in neck wall thickness is theoretically possible among manufacturers of that cartridge that stick to SAAMI standard dimensions. A standard die is designed to resize them all, including the ones that are -0.008" smaller in diameter. In order to handle those thin ones, it resizes all the others more than necessary, allowing the expander to bring the ID's all back up to the right value. This means the neck brass gets overworked twice: once by over-resizing and a second time by the expander makes it bigger again. Since most cases are not minimally thick, standard dies overwork most case necks.
One solution is to just size them the right amount in the first place. Your die may have come with a bushing that mimics the standard die (check with the maker) but average cases tend toward the middle of the tolerance range, so a bushing about +0.004" over the size of the standard die neck diameter should size the average case just about right in the first place. If you do, then dragging it over the expander (and it will drag because the expander will be about 0.001" bigger than the desired ID to allow for spring-back) serves no purpose. Indeed, the expanders can and do pull necks a little off-axis with the rest of the case, which works against achieving the smallest group sizes. So if you can leave it out, that's best. The alternate decapping pin chuck lets you do that.
Another method some use is to replace the expander with one from the next smaller caliber. This way there is still enough expander to remove dents from case mouths, but not enough to drag on the resized neck.
