IF this is what it is (work hardened brass)... every time you fire and resize a piece of brass you work harden it. Stretching and compressing the metal makes brass hard. It also changes it's "spring", meaning how much the brass springs back after firing or resizing. Your brass that has only been fired once is softer in the neck and shoulder area and is more malleable. The brass that has been fired 5 times is now harder and will have more spring to it, so as you... for a lack of a better word, squish it into the die and remove it the brass will "spring back" more than the once fired brass.
This is just my theory, and at this point it is only that... a theory. We need some guys in here with some other ideas.
To even all the brass back out you can anneal it (soften) with heat. I can give you a link to a gentleman that has an annealing machine that will clean and anneal your brass for a fair price. You ship it to him dirty and he decaps, cleans and anneals your brass and ships it back to you. Then you resize it and you should be golden. This will also prolong the overall life of your brass and reduce chances of split necks.