Could be the flavoring used has some sort of need for ratio changes when mixed in larger quantities. Like say 5ml of vg takes 2 drops of a particular flavor then if your making 30 ml the ratio is different for the same taste. Not 12 drops as the ratio of the recipe would be but maybe more than that due to larger volume. Just guessin dont diy myself but was a thought of a possible scenario.
If you're not DIY, you shouldn't have to deal with these problems. It's the
juice maker's responsibility to mix it right regardless of bottle size, and they charge you for that service. For bigger shops, I'd bet they mix it in large batches and simply squirt into 5, 10, 30, 50 ml bottles. Can you imagine someone mixing 10-30 mls at a time? They'd be mixing every waking hour. There are some pic/vid at Johnson Creek you can check out on their site or YouTube. If a vendor cannot sell me a consistent product, I would not go back.