Good grief, there are some bad answers here.
The choice of 19 volts is because is it comfortably below 20 volts which is the maximum output voltage of power supplies that can be certified as LPS (Limited Power Source) with non-inherent power delivery limits. If you can keep at or below 20 volts, the whole safety certification thing becomes easier and cheaper. To make sure you're within the limit accounting for manufacturing tolerances, go 5% lower, which is 19 volts. There you are. It has nothing to do with battery pack organization or LCD screens.