The Docking station comes with a 4.8v 600 mAH (four 1.2 NI-MH cell) battery pack. There is a separate 3-wire connector for this battery pack, so it might not just be wired in parallel with the throw-way battery terminals.

Remember too that in general it's not just battery voltage but the internal resistance of the battery which matters. This determines how much the voltage decreases with a given current draw.