Thread: Weight difference

I am reloading 6.5 creedmore, hornady brass, 129 sst, 38g 2209, cci primers. I have trimmed brass and neck sized shells and cleaned primer pockets.
I though I would weigh check fully loaded ammo. Now what I have found is there is a large variance between of ammo by as much as 6gr.
I then pulled bullets from the heaviest and lightest and checked the powder weights which were both the same 38gr
The only thing I never did before reloading was polish brass.
What would cause the variance in weight. All I can think of is powder residue or shell thickness.
Your thoughts