Can someone possibly explain this to me?

[Austin Albright]

Are you measuring your headspace on these fired pieces of brass with the spent primers still installed? I have found I get much more consistent measurements if I knock the spent primers out with a universal de-capping die.

That was going to be my suggestion too. Make sure the primers are out before measuring. Drove me craze until I did that.

 

[Charlie98]

Neck variation can change your lengths if using an expander ball.

+1 That drove me crazy for a while...