Tried digging into this a bit more, but haven't found an explicit reason why Tesla's low voltage battery is labelled 12v, when actually it is closer to 15v. Need an automotive electrical engineer to shed more light on this. My best guess is since the automotive supply industry uses a low voltage standard of 12v, the label is showing the battery's purpose rather than its true voltage. Note, for an ICE car with a lead acid battery, although the battery is closer to 12v, once the engine is started and the alternator kicks in the voltage going, to say, the headlights is higher, so headlights are designed to run on 13.2v
(see source here). So presumably the various 12v components have a tolerance range for the voltage levels they can accept. Tesla controls what 12v components go into their vehicles, and hence can ensure they can run on slightly higher voltages, but they can't control the 12v components that are in a trailer hooked up to their car, so that might explain articles like this one
Drive Tesla Canada: Trailer 12V auxiliary power disabled in recent tow hitch-equipped Model Ys due to upgraded 12V Li-Ion battery.
Munro Live interview with Elon Musk Feb 2, 2021 Talks about moving away from the existing 12v standard to say a 48v standard, which allows the use of smaller copper wires, giving both weight savings and cost savings, it could open up the possibility of combining the separate data and power wires into one set of wires, similar to powered ethernet. Also discusses the advantages of the Lithium ion battery.
YouTube: Munro Live: Sandy talking about Tesla's Lithium Ion battery - Jan 28, 2022 - Battery is from a Model S Plaid but it looks the same as the one on the Y. Someone has written 15.4 on it, which might be the voltage they measured on it. Sounds like we are going to have to wait for a video later in the tear down for a discussion of this battery.
InsideEVs: Tesla's New 12V Li-Ion Auxiliary Battery Has CATL Cells Inside Article sourcing its information from two Youtube videos of a destructive tear down of Tesla's Lithium-Ion battery by Ingineerix.
Ingineerix: Tesla Plaid - 12 volt Lithium Battery PART 1 At 9:53 says the battery voltage is much higher than a normal 12v system, fully charged it is around 16v, but still within the range most 12v accessories will work with. If incandescent light bulbs were still being used in Teslas then the bulb life would be shortened by this higher voltage.
Ingineerix: Update! Part 2 - Tesla Plaid 12v lithium battery. Shows a CATL spec sheet showing the nominal cell voltage is 3.7v, with a max of 4.2v. The battery contains 4 cells, so 14.8v nominal, 16.8v max.