A mate of mine just swapped the alternator on a "modern" jag. When he starts it and leaves it running, the voltage at the terminals is 15.5, then it suddenly drops to 13.5 after five minutes or so. This cycle repeats when he stops and re-starts it. He wandered if there was something "clever" in the regulating circuit to give it a quick boost every time the starter was used.
Sounds plausible, but I bet someone here knows!