There is a difference. I'm betting the 'aftermarket' tach reads "power on" as time. Our trucks do not start 'engine time' until the engine is running. Go give it a try. Turn your key switch on and check the hours, turn off key switch. Now, turn the key back on(do not start truck) and walk away for an hour. Tell me what the hours are when you get back. They should be the same as before.
If I can get well over 10k hours out of a GM 350 gas motor then these Diesels should double that, at least.