That's what a voltage regulator does, regulates voltage regardless of load. A charged battery presents a low load.
With the engine off in a system in good shape will read about 12 volts. If the alternator and regulator are working properly, with the engine running, you will read something in the area of about 13.5 volts or maybe a little more. After discharging the battery a little, the voltage may read around 14 volts, the voltage is increasing to recharge the battery.
Are you having a specific electrical system problem? If so, give more information and I'm sure we can help further.