It's not the press or dies that cause the variance. It is the shape of the bullets.
It's due to where the seater stem touches the bullet. The ogive (first part wide enough to touch rifling) is not in the exact same place on every bullet.
This is still ok and accuracy won't suffer due to this factor alone.
Imagine 10 men all 6' tall, but with different shoulder heights. The die is seating by the shoulders, not the top of the head.