That is figuratively speaking... 6mg/.6% on a .5ohm coil at say 50watts is about the equivalent of 12mg/1.2% on a 1.5ohm coil at 15 watts.
Basically what it boils down to is the lower the ohm of your coil + the higher the wattage you vape it at = a higher concentration of nic per inhale. The more vapor you inhale, the higher the nic content. Most people who try to sub ohm vape 12mg/1.2% or higher get "nic'd out" very quickly. So the majority of the vapers in the community never subohm over 6mg/.6%.