New to me too. The voltage is determined by the chemistry of the battery. That’s why you get 1.2V out of NiMH or NiCad batteries, 1.5V out of alkaline batteries, and 3.7V out of lithium ion batteries. The way this battery gets 1.5V is to incorporate a small buck converter to reduce the voltage from 3.7V to 1.5V. That’s also why the voltage is stable until it reaches the point where the lithium ion battery inside no longer delivers enough voltage to power the buck converter and it stops working until recharged. Seems like a rather expensive way to power a clock though. Even a cheap alkaline battery bought three or four for a dollar at my local dollar store will power my clocks for 6 months between battery changes.
BTW if you use a single 3.7V lithium battery to replace a pair of 1.5V batteries for 3V be sure to use the correct size resistor, not diode to drop the voltage to 3V. In many cases a single 18650 battery works fine to power a device using three 1.5V batteries. I did that with a bunch of lighted Halloween pumpkins recently.