A dyno, like a flow bench can not lie. But the operator can. So can the car owner.
I cut a letter to the editor out of a magazine back in 1983 when I was in the Army. I carried that thing with me for decades. I'd love to find it and make a copy of it. It was called "11 Second Mouths" and I had many a drunk idiot read it out loud at parties and such. It stopped quite a few fights. It pointed out how many guys have no idea how much power they have or even what their junk runs.