Is your PT regulated? If it isn't, how are you powering it?
If you are not using a 5 volt 2 amp wall wart to power it, then you probably aren't getting nearly as much power to the atomizer as you might think you are. So the regulated box mod would be providing more wattage to the atomizer than the PT.
Just guessing here, I'm sure someone much smarter than me will give you a better answer.