First off, make sure your measurements are accurate. I am assuming the voltages you reported were measured with an accurate voltmeter, and not the old gauges?
Now, back to basics. The charging output of the alternator is controlled by a voltage regulator, either internal or external. Since the 1980s most voltage regulators are solid-state and use transistors to control charging output.
The actual output voltage produced by the alternator will vary depending on temperature and load, but will typically
be about 1.5 to 2 volts higher than battery or battery bank voltage that the regulator 'sees' at its connection to the battery. At idle, most charging systems will produce between 13.8 to 15.3 volts with no loads on the system. This is measured by connecting the positive (+) and negative (-) test leads of a voltmeter to the battery posts while the engine is running.
Most alternators that are charging properly should produce a voltage of about 13.8 to 14.2 volts at idle with no loads on the system. Always refer to the manufacturer's literature to be sure. Some Asian systems have higher charging voltages of up to 15 volts.
When the engine is first started, the charging voltage should rise quickly to about 2 volts above the base battery voltage, then taper off, leveling out at the specified voltage.
The exact charging voltage will vary according to the battery's state of charge, the load on the electrical system and the temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The 'normal'
charging voltage on a typical application might be 13.9 to 15.1 volts at 77 degrees F. But, at 20 degrees below zero the charging voltage might be 14.9 to 15.8 volts. On a hot engine in a hot engine room, the normal charging voltage might drop to 13.5 to 14.3 volts. These numbers are all based upon fully charged batteries in good condition. You can see how important it is to have a reliable, accurate voltmeter to troubleshoot the system.
You really need to get the manufacturer's specifications for your specific alternator, and regulator if it is external. Just do a Google search for the make and model numbers and I'm sure you will quickly find the information you are looking for. The engine owner's manual is another good source of information.