It is a 12V DC unregulated power supply, with just a transformer, rectifying diodes and a bulk reservoir capacitor.
Such a power supply would read about 16V when measured without load.
12VRMS AC has peak voltage of almost 17V. That is rectified into a capacitor through one or two diodes so it will drop by some amount, which including some tolerances would be exactly the 16V DC you happen to measure.
Why it is an unreglated instead of something else? It likely is cheaper to have a single type of transformer which fits to many kind of devices. Devices have the regulators for the required voltages inside.
Another thing is, if it were a regulated supply, it would likely be a switch mode power supply that might be more difficult in an environment with audio devices being connected together. Having a standard cheap transformer generates less electromagnetic interference than cheap switch mode power supplies.