I can't believe this has made it 38 posts without someone saying this. Shame on us all.
EVERY TIME YOU CHANGE A COMPONENT YOU HAVE TO RE-PROOF YOUR LOAD BY STARTING A FEW GRAINS LOW AND WORKING BACK UP.
The variation the OP reports has been documented in multiple other threads. I have even reported it myself. It's well known, and even accepted at this point. It proves the NEED to check your components, prior to loading up your regular load and heading to the range. It is not fashionable, or healthy, to wear your rifle on your face.
I agree with you. My best WAG is perhaps it's inherent for most of us already. Also, in essence Kmccord addressed it in #5. I am never in hurry when reloading so I do 100% quality control check on each process for each round. I also log all pertinent information of my loads in my tracking log like most of us.