Are you in VV mode or VW mode?
If you are using variable voltage then with a fresh battery at say 4v, it will be a different output from your battery as the battery itself drain. This is where ohms law comes in to play.
If you use variable wattage mode, your microprocessor will do all the work for you and apply the math to whatever the battery status is at the time and adjust the output so this will always be the set wattage until your battery is drained.
So if your set at say 9W, you will get a consistent vape all day long