If you premix pg/vg/nic, you should know what flavor % you will be adding and create your premix stronger in such a way that the final mix will be your desired strength.
My point being that if I had a juice line I would not have seperate premixed blends of nic, pg and vg for every flavor. So in theory some juices may end up having a tad more pg than others, and subsequently less nic.