Bullet wobble or neck wobble.

So its how straight your ammo is. When you fire does your bullet come out cockeyed? Or straight into the rifling? You want it to be straight of course.

If you necks are too thick on one side or the other, bullet runout will occur. Your sizer can knock your case necks out. So when you go to seat the bullet, it won't go in straight. Or your seater die could be putting the bullet in cockeyed.

Its a long involved process to get perfectly straight ammo.