Matter of preference - there doesn't HAVE to be...however:
- if using a "dripper" people often are seeking MORE vapor, and can use thicker liquid - more VG than PG as the base of the liquid. That's why you'll see "Max VG" liquid - it's thicker, makes more vapor, but won't wick as easily in most tanks...
- ALSO if you're making your own coils and/or using lower resistance (like "subohm") then the nicotine gives you a bigger kick, and people usually drop the nic level way down. Common to vape 12 or 18 mg in a carto-tank or even a Kayfun, but then go to 4mg or 6mg in a dripping atomizer.
Hope that helps!