I was curious about that myself. I see where he is coming from in that the math on pg/vg and nic content would get all messed up if they simply doubled the amount of flavor. I have already emailed the supplier though to see if they would clarify it for me.
Here's the deal you learn by DIY, there is a range that flavorings "work". Below that range and it doesn't even taste anything like it is labeled. Above that range, same thing. Depending on flavoring that range can be as little as 5% from too little to too much. So I can double the flavor by only adding 3% more. Now if my flavoring works well at 10% of total volume, a 13% flavoring mix could very well be twice as strong.
Now, many flavorings are not that sensitive, but a number of them are.
OK, let's say you want to market your stronger tasting
juice, what would you call it? 3% more flavoring? Nah, doesn't ring, nobody would
buy that a tiny bit doubles their flavor... Would you go for it marketed that way?