Just as I mentioned, your sigelei is keeping you at about 30W. Power (watts) is voltage squared divided by resistance, or you can also calculate it as amps times voltage. Looking at the first formula, If you want to keep your power constant, and you lower your resistance, then the voltage has to go down too. If you raise the resistance, you can raise the voltage to get equal power. The numbers you're giving vary slightly in power but it's not a big difference, guessing it's just rounding errors in the chip.
The main critical thing I'm not sure you're seeing is that the amps and volts at the coil doesn't have to equal the amps and volts at the battery. You can transform between volts and amps, it's the same thing as running a high voltage line to your house and then getting only 110V out of the socket. The regulator chip is transforming between volts and amps. What remains consistent is the power, not the amps. Technically the power drops a little after transforming because of inefficiency but for simplicity you can think of the power remaining constant.
So in your example if power at the coil is 100W (2V and 50A) then to find out the battery current you solve using the power and voltage at the battery. So 100W with mostly charged batteries at 4V each, in series, would give 100W/8V = 12.5A at the battery. The regulator is transforming between 12.5A and 8V at the batteries to 50A and 2V at the coil. But the batteries don't care that it's 50A at the coil, they only see the 12.5A.
The 4V, 3.8V, 2V, etc that is shown on the display is the coil voltage, not the battery voltage. The battery voltage is independent of the voltage setting and therefore the battery current only depends on how much juice you have left in your batteries (the battery voltage) and the power setting of the device (the watts).
Does that make more sense?