The die doesnt touch every bullet in the same spot when pushing it into the case since every bullet is not made exactly the same. The ogive is the first part of the bullet that is big enough around the touch the rifling as it is fired in the barrel. Most seating dies have a cone or tapered seating plug so they don't push on the end of the bullet but mybe a 1/16" down from the tip. The part of the bullet that touches the seating plug could be higher or lower on each bullet thus your different lengths. Imagine 5 people all with shoulders 5'2" high, but each person with a slightly longer neck or head so their total height is not the same (6'1", 6'2" etc)
If you had a die that seated by the end of the bullet they would all be the same over all length, but then the ogive of the bullet would not be in the same place on them all. I would just seat them as you do without adjusting the die and they will shoot fine. If you need to compare there are the tools mentioned above but they are not needed by most loaders.
