Are you measuring your cases with a bump gauge. If you've gone to the expense of using the competition shellholder set, you really should be measuring your cases.
Also, how much resistance do you have on the down stroke after you've sized the case? If you aren't getting proper lube the length of the case, the die will grab the sides and pull/stretch the case longer.
I'm not saying you fall into this group of people but I've seen many reloaders that don't lube the case all the way to the rim, which is where it resizes and squeezes the case back down the most. If the case doesn't have any lube there it can grab, pull, and make the case actually longer.
If it's none of the above, you likely don't have you die screwed down far enough. But, if you aren't already using a bump gauge, you really need to get yourself one. Without measuring the cases, it's all guess work.
