The efficiency of a regulated mod is part dependent of the electronics inside, part dependent of the voltage. If the resistance is changed by choosing a different coil build, then the mod automatically adapts to this by supplying a different voltage, visibly, shown on the display of the mod.
The output voltage is not to be confused with the input voltage. The input voltage is the voltage that the board receives from the combined cells (the sum of each cell's voltage if the cells are hooked up in a series connection scheme), and, the voltage that a cell delivers is determined by the amount of charge remaining and the voltage sag of the cell in question. The voltage sag depends on a number of factors such as the current load placed on the cell and the characteristics of the cell, among several other factors. The run time is also affected by how the efficiency of the cell diminishes with increasing current load.