Engine hour meter

[hperros]

My engine hour meter underestimates the time the engine is on. It was replaced two years ago before I bought the boat, and it has no more than 200 hours on it. Can somebody please explain to me how it works? What causes it to turn on/off? In my previous boat it worked off the ignition key. As long as the ignition key was turned on, it ticked. Is it the same with the Universal?
Thanks