I think you have hit the nail on the head there, I spent ages checking all the setting and comparing the 2 mods but didnt think about the voltages applied to each.
Rookie mistake, the higher the coil resistance the higher the voltage needed for more power, It makes perfect sense. I have checked the two devices 0.34 ohms was getting 6.6V and the lower 0.21 ohms was getting 5.8V.
Im so used to concerning myself about the battery ampage limit forgot to think of the obvious.
Thanks for helping me see the correct point of view.