Re: How does a seating die work.
If you take the die apart, you will answer this question. The seating stem is hollow. The bullet nose fits into this space and then pushed into the neck of the case.
Since this diameter is in a different locations in different bullets, it is possible to seat different brands of bullets to different OAL.
If seating softpoint bullets of the same brand, different OAL also can occur due to the amount of exposed lead. Some noses get knocked shorter.
Measure OAL using the ogive of the bullet. Sinclair makes a neat tool that works with a vernier caliper. simple and accurate. You should find that the OAL measured this way is basically the same. If not, tighten the screws on the seating die and/or change bullets.