My cardio workout is pretty invariable. I go for an hour on an elliptical, adjusting the difficulty so my heartrate stays in a target zone. I go often enough so that if I jump on a machine and adjust the settings to stay in my zone, I'll get roughly the same calorie output (about 720) for an hour's effort. So I can tell variations among machines. For example, of the five ellipticals at Lloyd, I know which outputs the most, and least, calories per workout--but they're all within about 20 calories of each other (for my settings)--or about a 3% variation.
I've visited the Hollywood 24 Hour twice. Not only can I set the same model of elliptical there to a higher degree of resistance, but the output is about 80 calories above what I average for an hour of effort (about an 11% variation).
The other day I visited the 24 Hour in Mall 205, and found an older model of elliptical, so I couldn't compare degrees of resistance, but the readout was 300 calories above what I average for an hour of effort--as in, about 1,000 calories per hour. It didn't seem to be restricted to my machine, either: dude next to me was moving slower than I was, at lower difficulty settings, and his machine was telling him he was burning 800 calories per hour.
Obviously the result is what you see happening to your body, but I wonder what the adjustability limits are, or whether/when people are actually clueing in that gyms may be, essentially, trying to deceive them.