On my vehicle, with normal light-loading, the generator will maintain the specified voltage at idle. On my vehicle the voltage is higher at cold temperature and lower at warm temperature.
When the current load increases (lights, wipers, amplifier, etc...) the generator won't keep up at idle. But, the generator is specified to provide full output at the test speed of 1,250 RPM. That is where I set it for generator output measurement using my tools and instruments.
What does the FSM say about the generator on your vehicle? Which vehicle is affected?