One reason this might be is because vape shops tend to have a set PG/VG amount they use for all their flavors. For example, 80/20 of PG/VG or 60/40. They also have the different levels of nicotine mixed in with the PG and VG. If this is the case, which it might not be, then that would mean that the PG, VG, and nicotine are already steeped, so when you add that mixture to the flavoring, there's less acclamation taking place. So, instead of having the PG, VG, nicotine, and the flavoring mixed at the time of purchase, which would require all those elements to blend and marry well, you can treat the premixed PG, VG, and nicotine as one liquid versus three. Try it yourself to see if it makes a difference. You can use your own ratio of PG/VG and nicotine and mix it and allow to steep and then add flavoring after it's steeped and then compare it to the same ratio of PG/VG and nicotine by mixing all of the components individually. Hope this helps.