I have had a few when seated an measured with my hornady comparator thingy that were way off- .015-.o18 like the bullet was different...
I have had the same thing happen to me with RCBS, Hornady, Redding, Lee and Lyman dies. I have even had some like yours. I am some what annal and measure every bullet. What I do is a PITA but satisfies my need for regularity (no pun intended) is to seat every bullet a little short of intended OAL (Hornady comparator). Maybe 0.005" or so and then start sneaking up on the OAL a 0.001 - 0.002 at a time. As a loaded case reaches target OAL I put it in the box. If it is short I segregate according to length and keep seating.
I think that sometimes the bullet does not start out straight in the seating die and may catch momentarily on the edge of the seating stem which can drive them deeper. If the seater is one of the slider type such as Lee or Hornady, the slider may hang and produce a deeper seated bullet. I will clean and lightly lubricate these sliders. Good luck on your loading.
